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/run/RunSetup.tsx

@@ -43,6 +43,7 @@ import type { CwdSuggestion, CcMdItem } from "../../lib/api";
 import type { LaneTool } from "../../lib/types";
 import { Select } from "../Select";
 import { ContextMenu, MenuItem, menuPoint, type MenuPoint } from "../ContextMenu";
+import { isSubmitEnter } from "../../lib/keys";
 
 export interface SlashCommand {
   name: string;
@@ -131,6 +132,9 @@ interface PromptEditorProps {
   onImagesChange: Dispatch<SetStateAction<PromptImage[]>>;
   /** Borderless, for a composer box that draws its own frame. */
   bare?: boolean;
+  /** An image upload started (true) or settled (false): the caller holds its
+   *  send button meanwhile, or the prompt would go out without the image. */
+  onBusyChange?: (busy: boolean) => void;
 }
 
 /** An attached image: its path on the server's machine and a local preview. */
@@ -153,6 +157,7 @@ export function PromptEditor({
   images,
   onImagesChange,
   bare,
+  onBusyChange,
 }: PromptEditorProps) {
   const { t } = useTranslation("run");
   const taRef = useRef<HTMLTextAreaElement | null>(null);
@@ -265,6 +270,10 @@ export function PromptEditor({
   const [upload, setUpload] = useState<"uploading" | "failed" | null>(null);
   const [viewing, setViewing] = useState<PromptImage | null>(null);
   const [imageMenu, setImageMenu] = useState<(MenuPoint & { img: PromptImage }) | null>(null);
+  const uploading = upload === "uploading";
+  const busyRef = useRef(onBusyChange);
+  busyRef.current = onBusyChange;
+  useEffect(() => busyRef.current?.(uploading), [uploading]);
   const attachImages = (list: FileList | null | undefined) => {
     const picked = Array.from(list ?? []).filter((f) => f.type.startsWith("image/"));
     if (picked.length === 0) return false;
@@ -320,10 +329,11 @@ export function PromptEditor({
       return;
     }
     // Enter sends (Ctrl/Cmd+Enter still does); Shift+Enter inserts a newline.
-    // isComposing guards IME input (Vietnamese/CJK), where Enter commits the candidate.
-    if (e.key === "Enter" && !e.shiftKey && !e.nativeEvent.isComposing) {
+    // isSubmitEnter guards IME input (Vietnamese/CJK), where Enter commits the
+    // candidate. An image still uploading holds the send.
+    if (isSubmitEnter(e)) {
       e.preventDefault();
-      onSubmit();
+      if (!uploading) onSubmit();
     }
   };
 
@@ -878,7 +888,7 @@ export function ModelMenu({
               autoComplete="off"
               spellCheck={false}
               onKeyDown={(e) => {
-                if (e.key !== "Enter") return;
+                if (!isSubmitEnter(e)) return;
                 e.preventDefault();
                 setCustom(false);
                 pick(e.currentTarget.value.trim());