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/components/lanes/SessionResumeButton.tsx

@@ -7,8 +7,9 @@
  * (`forkSession: true` → `claude --resume <id> --fork-session`), leaving the
  * original untouched, and is offered even while the original has a live pane. A
  * session that still has a live pane offers that pane instead. A resume
- * starts in the workspace's first free pane slot (`POST /api/lanes/:id/start
- * {slot, resumeSessionId}`, panes re-read first) and opens it — offered only
+ * starts in the workspace's first free pane slot, picked by the server
+ * (`POST /api/lanes/:id/start {slot: "auto", resumeSessionId}`, panes re-read
+ * first) and opens it — offered only
  * for a session whose folder IS the workspace's, because the pane starts in
  * the workspace's folder and Claude finds a session by the folder it ran in.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
@@ -22,7 +23,7 @@ import { api, type RunHandle } from "../../lib/api";
 import type { Lane, Session } from "../../lib/types";
 import { useLanes } from "../../hooks/useLanes";
 import { ConfirmModal } from "../ConfirmModal";
-import { freeSlot, paneRunId } from "./WorkspaceHome";
+import { startInFreeTabDetailed } from "./startSession";
 import { liveRunFor, sessionWorktreeName } from "./WorkspaceSessionList";
 
 /** The workspace of this tool whose folder holds `cwd` most deeply — the
@@ -81,8 +82,8 @@ export function SessionResumeButton({
     (same(session.cwd, lane.cwd) ||
       sessionWorktreeName(lane, { cwd: session.cwd } as Session) !== null);
   if (!run && !resumable) return null;
-  const open = (runId: string) =>
-    navigate(`/workspace?lane=${lane.id}&run=${encodeURIComponent(runId)}`);
+  const open = (runId: string, laneId = lane.id) =>
+    navigate(`/workspace?lane=${laneId}&run=${encodeURIComponent(runId)}`);
 
   const resume = async (fork: boolean) => {
     // Busy first: the Confirm button is disabled only through it, and the
@@ -99,17 +100,12 @@ export function SessionResumeButton({
       setRuns(fresh);
       const live = liveRunFor(lane, fresh, session.id);
       if (live && !fork) return open(live.id);
-      const slot = freeSlot(lane.id, fresh);
-      if (slot === null) {
-        setError(t("home.allBusy"));
-        return;
-      }
-      const r = await api.lanes.action(lane.id, "start", {
-        slot,
+      // A pane of another (nested) workspace may hold it: open it there.
+      const r = await startInFreeTabDetailed(lane.id, {
         resumeSessionId: session.id,
         ...(fork ? { forkSession: true } : {}),
       });
-      open(r.run_id ?? paneRunId(lane.id, slot));
+      open(r.runId, r.laneId);
     } catch (err) {
       setError(err instanceof Error ? err.message : String(err));
     } finally {