Expand a leading ~, and say what the failing command said

Three things, two of which are the same failure seen from opposite ends.

**A working directory of `~/repos/ai-app` never worked.** Everything
crossing to the remote side is single-quoted, which is right for paths,
model names and prompts alike -- unquoted they would be shell syntax
rather than data. It is wrong for exactly one character: `~` means "expand
me", and quoting is what stops expansion. So the remote shell was handed
the literal four-character directory `~` and correctly said it did not
exist, which reads as the path being wrong rather than the quoting.

Paths now go through `quote_path`, which emits `"$HOME"` for a leading
`~/` and single-quotes the rest. The variable expands, the expansion is
not re-split or globbed because it is double-quoted, and nothing after it
gains a meaning -- there is a test that pushes a quote-and-semicolon
injection through the tilde branch and gets back one absurd path rather
than three commands. `$HOME` is set by every shell this can land in, so
this does not depend on the remote side being POSIX; verified by running
the generated script under both sh and fish, which is what the dev VM
actually uses.

**The phone could not have told you any of that.** The exit report kept
the last line of stderr, and a shell's error message ends with a blank
line -- so the last line was empty, the report was a bare exit status, and
the seven lines of fish complaining sat in the server's log where nobody
holding a phone is looking. It now keeps the last 50 lines in a ring and
reports them with blank lines trimmed from both ends. The tests use the
real fish `cd` failure as their fixture.

**The status bar was unreadable.** `isAppearanceLightStatusBars` was
hardcoded to `true` -- dark icons -- which was right against the default
light surface and wrong the moment the app wore Mocha. It now asks the
scheme's own background for its luminance, so changing the palette cannot
reintroduce it.

**And the address field takes `user@host:port`.** One field rather than
two, because that is how an address is written everywhere else and a port
that is nearly always 22 does not deserve its own box on a phone keyboard.
Absent means absent rather than 22: the backend already decides that
default, and writing it here would be a second answer in a second place.
A colon only means "port" when it can -- brackets for IPv6 as ssh writes
them, otherwise exactly one colon followed by digits.

Looked at on the emulator: the status bar, and the form, whose label I
then shortened because it wrapped onto a second line and made that field
taller than the two beside it.

server/src/session/claude.rs

@@ -34,6 +34,7 @@
 //! - `control_request{subtype:set_model}` answers success;
 //!   `{subtype:interrupt}` stops the turn.
 
+use std::collections::VecDeque;
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
 
@@ -47,6 +48,34 @@ use super::transport::{Launch, Transport};
 use crate::config::{ProviderConfig, SessionConfig};
 use translate::{AnswerOutcome, Translator};
 
+/// How much of a failing process's stderr the exit report carries.
+///
+/// Enough for a shell's complaint plus the context it prints around it --
+/// fish's `cd` failure is seven lines including a caret pointing at the
+/// offending line -- and bounded because this is held per session for the
+/// life of the process and a chatty program would otherwise grow without
+/// limit.
+const STDERR_LINES_KEPT: usize = 50;
+
+/// The kept stderr as one block, with blank lines trimmed off both ends.
+///
+/// The trailing trim is the point: a shell's error ends with a blank line,
+/// so the last line of stderr is routinely empty and anything that reports
+/// "the last line" reports nothing at all.
+fn tail_of(kept: &VecDeque<String>) -> String {
+    let lines: Vec<&str> = kept.iter().map(String::as_str).collect();
+    let start = lines
+        .iter()
+        .position(|line| !line.trim().is_empty())
+        .unwrap_or(lines.len());
+    let end = lines
+        .iter()
+        .rposition(|line| !line.trim().is_empty())
+        .map(|last| last + 1)
+        .unwrap_or(start);
+    lines[start..end].join("\n")
+}
+
 /// Where the driver remembers its CLI session id between backend runs --
 /// the whole crash-recovery story: respawning with `--resume <id>` picks
 /// the conversation back up from Claude's own session files. Kept in the
@@ -138,19 +167,31 @@ impl ClaudeDriver {
         ));
 
         // stderr is diagnostics only; surface it in the log, and keep the
-        // last line for the exit report below. For a remote provider this
+        // tail of it for the exit report below. For a remote provider this
         // is also where ssh's own failures arrive ("Permission denied",
         // "Could not resolve hostname"), which are the ones a person
         // actually needs to see.
-        let last_stderr = Arc::new(Mutex::new(String::new()));
+        //
+        // A ring of the last lines rather than the last line alone. Keeping
+        // one line meant keeping whatever happened to come last, and what
+        // comes last is very often blank -- a shell's error message ends
+        // with one -- so the report was a bare exit status and the actual
+        // complaint existed only in the server's log, which is not where
+        // the person holding the phone is looking. A failing `cd` cost an
+        // evening to exactly that.
+        let recent_stderr = Arc::new(Mutex::new(VecDeque::<String>::new()));
         {
-            let last_stderr = Arc::clone(&last_stderr);
+            let recent_stderr = Arc::clone(&recent_stderr);
             let label = provider.name.clone();
             tokio::spawn(async move {
                 let mut lines = BufReader::new(stderr).lines();
                 while let Ok(Some(line)) = lines.next_line().await {
                     tracing::warn!("{label} stderr: {line}");
-                    *last_stderr.lock().unwrap() = line;
+                    let mut kept = recent_stderr.lock().unwrap();
+                    kept.push_back(line);
+                    while kept.len() > STDERR_LINES_KEPT {
+                        kept.pop_front();
+                    }
                 }
             });
         }
@@ -172,16 +213,13 @@ impl ClaudeDriver {
                 if let Some(status) = status
                     && !status.success()
                 {
-                    let detail = last_stderr.lock().unwrap().clone();
+                    let detail = tail_of(&recent_stderr.lock().unwrap());
                     let _ = sink.send(Event::Error {
-                        message: format!(
-                            "{label} exited with {status}{}",
-                            if detail.is_empty() {
-                                String::new()
-                            } else {
-                                format!(": {detail}")
-                            }
-                        ),
+                        message: if detail.is_empty() {
+                            format!("{label} exited with {status}")
+                        } else {
+                            format!("{label} exited with {status}:\n{detail}")
+                        },
                     });
                 }
                 let _ = sink.send(Event::Status {
@@ -365,3 +403,47 @@ fn attachment_block(session_dir: &Path, id: &str) -> Result<Value> {
         }
     }))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The failure this exists for: a shell's complaint ends with a blank
+    /// line, so reporting "the last line of stderr" reported nothing, and
+    /// the phone showed a bare exit status while the reason sat in the
+    /// server's log.
+    #[test]
+    fn the_report_keeps_the_message_and_not_the_blank_line_after_it() {
+        let fish_cd_failure = [
+            "cd: The directory '~/repos/ai-app' does not exist",
+            "",
+            "embedded:functions/cd.fish (line 26): ",
+            "    builtin cd $argv",
+            "    ^",
+            "in function 'cd' with arguments '~/repos/ai-app'",
+            "",
+        ];
+        let kept: VecDeque<String> = fish_cd_failure.iter().map(|l| l.to_string()).collect();
+
+        let report = tail_of(&kept);
+        assert!(
+            report.starts_with("cd: The directory"),
+            "the complaint leads: {report}",
+        );
+        assert!(
+            report.ends_with("'~/repos/ai-app'"),
+            "the trailing blank is trimmed: {report:?}",
+        );
+        // The blank *between* lines is part of the message and stays.
+        assert!(report.contains("does not exist\n\nembedded:"), "{report:?}");
+    }
+
+    /// Nothing to say is said as nothing, so the caller can tell the two
+    /// apart and print just the exit status.
+    #[test]
+    fn stderr_that_is_only_blank_lines_reports_as_empty() {
+        let kept: VecDeque<String> = ["", "  ", ""].iter().map(|l| l.to_string()).collect();
+        assert_eq!(tail_of(&kept), "");
+        assert_eq!(tail_of(&VecDeque::new()), "");
+    }
+}