Select any of the transcript, and take a queued message back

Two things a reader could not do to what is on screen.

**Selection.** Nothing in the transcript was selectable at all, so a
command, a path or an error message could be read and not copied. One
`SelectionContainer` around the whole list rather than one per row: a
transcript is one body of text to a reader, and a selection has to be able
to run from a reply into the tool output under it. Per row it also could
not, and whatever was drawn without a container would have been silently
unselectable -- a state nothing on screen reports. Rows keep their tap
handlers; checked on the emulator that expanding a tool call, scrolling and
flinging are all unaffected, since a selection is a long press.

**Taking a message back.** A message sent into a running turn sits as a
bubble waiting to be read, and there was no way to change your mind: it is
tappable now, and the server answers `POST /sessions/{id}/unqueue`.

The answer has three states, and the middle one is the point. Claude's
driver writes a steer into the CLI's stdin the instant it arrives -- that
is what makes it reach the model at the next tool boundary rather than at
the end of the turn, and it was measured -- so the line is already gone and
`AlreadySent` is the only honest answer it can give. Holding the write
until a boundary would make the drop real and cost a steer one model call,
which is the latency the immediate write exists to remove; rejected on that
trade, with the reasoning in PLAN.md. The refusal is drawn on the bubble
that was pressed rather than in the error row under the header, a screen
away from it.

Where a driver really does hold its queue -- echo today -- the message goes
for good, and it goes as an `Event::MessageDropped` rather than as a return
value: every device watching the session loses the bubble, and a phone that
reconnects and replays the `messageQueued` does not put back one that was
cancelled with nothing left to resolve it.

server/src/session/mod.rs

@@ -32,7 +32,9 @@ use crate::config::{
     Config, DriverKind, ProviderConfig, SessionConfig, SetupConfig, SshConfig, TokenEntry,
 };
 use claude::ClaudeDriver;
-use driver::{Driver, Event, EventSink, ImageRef, SessionCommand, SessionStatus, context_after};
+use driver::{
+    Driver, Event, EventSink, ImageRef, SessionCommand, SessionStatus, Unqueued, context_after,
+};
 use echo::EchoDriver;
 use llama::LlamaDriver;
 use transcript::{SeqEvent, Transcript};
@@ -440,6 +442,21 @@ impl LiveSession {
         self.ask("be interrupted", |driver| driver.interrupt());
     }
 
+    /// Takes back a message the session has not read yet, named by the id
+    /// its `MessageQueued` carried. See [`Driver::unqueue`] for why the
+    /// answer has three states.
+    ///
+    /// A session with no process answers `Unknown` rather than being
+    /// reported as a failure, and that is the true answer: a driver on its
+    /// way out already said what it was holding (`Queue::close`), so there
+    /// is nothing waiting to take back.
+    pub fn unqueue(&self, message_id: &str) -> Unqueued {
+        match self.driver() {
+            Some(driver) => driver.unqueue(message_id),
+            None => Unqueued::Unknown,
+        }
+    }
+
     /// Leaves this session's process running and stops attending to it,
     /// for a server that is going away and means to come back. See
     /// [`Driver::detach`].
@@ -2716,6 +2733,86 @@ mod tests {
         );
     }
 
+    /// A queued message can be taken back until the driver has handed it
+    /// over, and the taking back is an event rather than a return value --
+    /// which is what makes the bubble disappear on every device watching,
+    /// and stay gone when one of them reconnects and replays.
+    ///
+    /// Exercised on echo because echo really holds its queue. The Claude
+    /// driver writes a steer into the CLI the moment it arrives, so it can
+    /// only ever answer `AlreadySent`; the case where a drop *succeeds*
+    /// has no other driver to be tested against.
+    #[tokio::test]
+    async fn a_queued_message_can_be_taken_back_until_the_session_has_it() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        seed_echo_only(&dir.path().join("config.ron"));
+        let manager = SessionManager::new(
+            dir.path().join("config.ron"),
+            dir.path().join("sessions"),
+            dir.path().join("models"),
+        )
+        .expect("manager");
+        let info = manager.spawn_session(echo_spec()).expect("spawn");
+        let session = manager.session(&info.id).expect("live session");
+        let mut rx = session.subscribe();
+
+        // A turn long enough that the next message has to wait behind it.
+        session.send_message("/slow 1".to_string(), Vec::new());
+        collect_until(&mut rx, |event| {
+            matches!(
+                event,
+                Event::Status {
+                    state: SessionStatus::Running
+                }
+            )
+        })
+        .await;
+
+        session.send_message("second thoughts".to_string(), Vec::new());
+        let seen = collect_until(&mut rx, |event| {
+            matches!(event, Event::MessageQueued { .. })
+        })
+        .await;
+        let Some(Event::MessageQueued { id, .. }) =
+            seen.iter().map(|entry| entry.event.clone()).next_back()
+        else {
+            panic!("expected the message to be queued: {seen:?}");
+        };
+
+        assert_eq!(session.unqueue(&id), Unqueued::Dropped);
+        let seen = collect_until(&mut rx, |event| {
+            matches!(event, Event::MessageDropped { .. })
+        })
+        .await;
+        assert!(
+            seen.iter().any(|entry| matches!(
+                &entry.event,
+                Event::MessageDropped { id: dropped } if *dropped == id
+            )),
+            "the drop has to be recorded, not merely returned: {seen:?}"
+        );
+
+        // Gone for good: the turn ends without the message ever entering
+        // the conversation, and asking again says there is nothing there
+        // rather than dropping it twice.
+        assert_eq!(session.unqueue(&id), Unqueued::Unknown);
+        let seen = collect_until(&mut rx, |event| {
+            matches!(
+                event,
+                Event::Status {
+                    state: SessionStatus::Idle
+                }
+            )
+        })
+        .await;
+        assert!(
+            !seen
+                .iter()
+                .any(|entry| matches!(entry.event, Event::UserMessage { .. })),
+            "a message taken back must never be read: {seen:?}"
+        );
+    }
+
     #[tokio::test]
     async fn a_command_on_an_idle_session_goes_straight_out() {
         let dir = tempfile::tempdir().expect("tempdir");