Show a replayed session's images instead of dropping them

An imported session showed no screenshots. `text_of` kept only `text`
blocks, so every image in the replayed tail was silently discarded -- while
the *live* translator has always saved them into the session's `files/` and
referenced them. Two readings of the same records, and the one used for
history was the lesser.

`save_image` moves out of `Translator` to a free function both paths call,
since the naming scheme for that directory should exist once. `events_from`
now takes the session directory to write into, which means the conversion
has to happen where that directory exists -- so `Seed` carries the raw
JSONL and `launch` turns it into events, rather than `routes` doing it
before the session is created.

Costs nothing in tokens, which is the point worth recording: this writes
into ai-app's own session directory and the phone fetches a reference only
when it draws one. Nothing here is ever written to the CLI's stdin -- it
reads its own session file, and the only things this app sends it are typed
messages, control requests and `/compact`.

Verified against the 133 MB session behind the 2026-08-29 incident: 45
images in the replayed tail, written as real PNGs and served over the files
route, with the transcript itself staying at 756 KB of references.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VETa8afmpWaYezLCqJhDB8

server/src/routes.rs

@@ -478,7 +478,7 @@ async fn spawn_session(
                      whole thing. Close it there first, then import it here."
                 )));
             }
-            let events = crate::session::import::replay(&transport, &chosen.path)
+            let records = crate::session::import::read_tail(&transport, &chosen.path)
                 .await
                 .map_err(bad_request)?;
             // The recorded directory can outlive itself; resuming into one
@@ -495,7 +495,7 @@ async fn spawn_session(
                 );
                 chosen.cwd = String::new();
             }
-            Some((chosen, events))
+            Some((chosen, records))
         }
         None => None,
     };
@@ -532,11 +532,11 @@ async fn spawn_session(
     };
 
     let info = match seed {
-        Some((chosen, events)) => {
+        Some((chosen, records)) => {
             tracing::info!(
-                "importing Claude Code session {} ({} events replayed)",
+                "importing Claude Code session {} ({} lines replayed)",
                 chosen.id,
-                events.len()
+                records.lines().count()
             );
             manager.spawn_imported(
                 spec,
@@ -549,7 +549,7 @@ async fn spawn_session(
                         path: chosen.path,
                         lines: chosen.lines,
                     },
-                    events,
+                    records,
                 },
             )
         }