feat(terminal): snapshot-replay on tab switches (xterm serialize + live pane capture)

Switching away from a session and back replayed only the server's byte
history. For TUI modes (codex especially) that shows just the latest
repaint — the idle banner — because the TUI drops earlier conversation
from its current frame. This restores the actual on-screen view.

Two complementary mechanisms:

- Client: load xterm's SerializeAddon and snapshot the rendered state
  (viewport + scrollback + colors) per session on switch-away, restoring
  it for an instant first paint on switch-back. The snapshot is only the
  first paint — the canonical /terminal frame is still fetched and
  reconciled (restoredSnapshot/clearedForBusy force the replay). Snapshots
  are LRU-bounded in memory (<=20) and persisted to localStorage
  (<=256KB each, <=10 sessions, stale-pruned) so they survive tab discard.

- Server: GET /api/sessions/:id/terminal prepends the live tmux pane
  buffer (via the existing captureActivePaneBuffer) ahead of the byte
  history, cleared between, so replay reflects the current frame.

Also fix formatPaneSnapshot dropping the rightmost column of every
captured row: it painted to cols - 1 out of caution about last-column
autowrap, but every row is followed by an absolute cursor-position CSI
that cancels xterm's pending-wrap, so painting the full width is safe.

The SerializeAddon is built from @xterm/addon-serialize (new dependency)
into the vendor bundle by postinstall.js (dev) and build.mjs (prod),
matching how the other xterm addons are vendored.

src/web/routes/session-routes.ts

@@ -932,7 +932,24 @@ export function registerSessionRoutes(
     const query = req.query as { tail?: string };
     const session = findSessionOrFail(ctx, id);
 
-    const rawBuffer = session.terminalBuffer;
+    // Prepend the live tmux pane buffer so tab-switch replay shows the current
+    // on-screen frame, not just the accumulated byte history. This matters for
+    // TUI modes (codex/opencode) that repaint only their latest frame: the
+    // accumulated buffer alone replays as the idle banner. We clear the viewport
+    // (`\x1b[H\x1b[2J`) between the history and the live pane so they don't
+    // overlap. `captureActivePaneBuffer` is a no-op ('') under test mode and
+    // returns null when unavailable, in which case we fall back to history.
+    const muxName = session.muxName;
+    const liveMuxBuffer =
+      muxName && typeof ctx.mux.captureActivePaneBuffer === 'function'
+        ? ctx.mux.captureActivePaneBuffer(muxName)
+        : null;
+    const rawBuffer =
+      liveMuxBuffer !== null && liveMuxBuffer.length > 0
+        ? session.terminalBufferLength > 0
+          ? `${session.terminalBuffer}\x1b[H\x1b[2J${liveMuxBuffer}`
+          : liveMuxBuffer
+        : session.terminalBuffer;
     const tailBytes = query.tail ? parseInt(query.tail, 10) : 0;
     const fullSize = rawBuffer.length;
     let truncated = false;