feat(workspace): no double resume, server-picked tabs, terminal a11y

- A resume of a session a live pane already holds is refused (409
  ESESSIONLIVE with error.run_id) and the client opens that pane, pasting
  any typed message there; catalog tools refuse resumeSessionId
  (EUNSUPPORTED) instead of silently starting fresh.
- POST /api/lanes/:id/start takes slot: "auto": the server picks the free
  tab under the lane lock (components/lanes/startSession.ts on the client,
  kept out of WorkspaceHome so the sidebar does not pull react-markdown
  into the main bundle).
- Unsaved whole-file edits are guarded on every exit (Back, another file,
  tab switch, close, lane/session change, beforeunload).
- Closing an AI tab asks first; hiding a pane has its own icon. Tabs and
  pane headers show the session name, a state dot, rename by double-click.
- Terminal: Ctrl+M toggles Tab-moves-focus (no keyboard trap), Cmd+F /
  Ctrl+Shift+F find bar, clickable links (noopener), a write-only OSC 52
  clipboard, a Reconnecting overlay, debounced resizes, optional screen
  reader mode. Alt+1-4 / Alt+\ / Alt+W focus, split and hide panes.
- Changes panel: draggable splitter, wrapping git bar, history paging,
  disarming discard/undo/abort, inline diff comments pasted only into a
  visible pane; drag MIME guard on sidebar rows; IME-safe Enter; Show
  more in the sidebar and Recents; idle dots readable with sr-only text.
- Adds @xterm/addon-clipboard, @xterm/addon-search, @xterm/addon-web-links.

client/src/lib/panePaste.ts

@@ -4,18 +4,30 @@
  * mounted TerminalView registers its run id; the text goes to the pane of that
  * workspace the user focused last, else any of its panes. It is a paste —
  * bracketed, so `claude` takes it as one block in its composer — and never an
- * Enter: the user reads it and sends it.
+ * Enter: the user reads it and sends it. Only a pane on SCREEN takes it (one
+ * hidden behind a full-width Changes panel or a replay would swallow it
+ * unseen); the caller says where it went, or that it went nowhere.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-const panes = new Map<string, (text: string) => void>();
+interface Pane {
+  paste: (text: string) => void;
+  visible: () => boolean;
+}
+const panes = new Map<string, Pane>();
 let lastFocused: string | null = null;
 
-/** Registers a mounted terminal; returns its unregister. */
-export function registerPane(runId: string, paste: (text: string) => void): () => void {
-  panes.set(runId, paste);
+/** Registers a mounted terminal (`visible`: is it on screen now); returns its
+ *  unregister. */
+export function registerPane(
+  runId: string,
+  paste: (text: string) => void,
+  visible: () => boolean = () => true
+): () => void {
+  const entry = { paste, visible };
+  panes.set(runId, entry);
   return () => {
-    if (panes.get(runId) === paste) panes.delete(runId);
+    if (panes.get(runId) === entry) panes.delete(runId);
     if (lastFocused === runId) lastFocused = null;
   };
 }
@@ -30,13 +42,21 @@ function ofLane(runId: string, laneId: number): boolean {
   return runId === base || runId.startsWith(`${base}-`);
 }
 
-/** Pastes `text` into one of the workspace's mounted panes; false when none is. */
-export function pasteToLane(laneId: number, text: string): boolean {
-  const id =
-    lastFocused && panes.has(lastFocused) && ofLane(lastFocused, laneId)
-      ? lastFocused
-      : [...panes.keys()].find((k) => ofLane(k, laneId));
-  if (!id) return false;
-  panes.get(id)!(text);
+/** Pastes `text` into the pane of `runId` when it is mounted and on screen;
+ *  false otherwise (not mounted yet, or hidden). */
+export function pasteToRun(runId: string, text: string): boolean {
+  const p = panes.get(runId);
+  if (!p?.visible()) return false;
+  p.paste(text);
   return true;
 }
+
+/** Pastes `text` into one of the workspace's VISIBLE panes — the one focused
+ *  last when it is — and returns its run id; null when none is on screen. */
+export function pasteToLane(laneId: number, text: string): string | null {
+  const ok = (id: string) => ofLane(id, laneId) && !!panes.get(id)?.visible();
+  const id = lastFocused && ok(lastFocused) ? lastFocused : [...panes.keys()].find(ok);
+  if (!id) return null;
+  panes.get(id)!.paste(text);
+  return id;
+}