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/LaneDiffView.tsx

@@ -18,12 +18,16 @@
  * so every windowed row is pinned to `ROW_PX` and the range is pure arithmetic
  * (`visibleRange`); the content's min-width comes from the longest line, so
  * the horizontal extent does not jump as rows come and go.
+ * A line's comment box floats just under the clicked line number (fixed,
+ * re-placed as the diff scrolls), not under the whole diff.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
 import {
   createContext,
+  useCallback,
   useContext,
+  useEffect,
   useMemo,
   useRef,
   useState,
@@ -265,7 +269,7 @@ export function analyseHunk(hunk: LaneDiffHunk): Analysed {
 /** Set by a LaneDiffView given `onComment`: a click on a NEW line's number
  *  opens the comment box for that line. */
 const CommentLine = createContext<{
-  open: (line: number) => void;
+  open: (line: number, anchor?: HTMLElement) => void;
   title: (line: number) => string;
 } | null>(null);
 
@@ -276,7 +280,7 @@ function Gutter({ n, commentable = false }: { n: number | null; commentable?: bo
       <button
         type="button"
         data-testid={`diff-comment-line-${n}`}
-        onClick={() => comment.open(n)}
+        onClick={(e) => comment.open(n, e.currentTarget)}
         title={comment.title(n)}
         className="w-10 shrink-0 select-none px-1 text-right text-fg-muted/60 hover:bg-accent/20 hover:text-accent"
       >
@@ -741,18 +745,34 @@ export function LaneDiffView({
   const [commentAt, setCommentAt] = useState<number | null>(null);
   const [draft, setDraft] = useState("");
   const [undelivered, setUndelivered] = useState(false);
+  // The clicked line number: the box floats just under it, where the eye is,
+  // instead of under the whole (possibly 2000-line) diff.
+  const anchorEl = useRef<HTMLElement | null>(null);
+  const [anchor, setAnchor] = useState<{ top: number; left: number } | null>(null);
+  const place = useCallback(() => {
+    const r = anchorEl.current?.isConnected ? anchorEl.current.getBoundingClientRect() : null;
+    if (r) setAnchor({ top: r.bottom + 2, left: r.left });
+  }, []);
+  useEffect(() => {
+    if (commentAt == null) return;
+    window.addEventListener("resize", place);
+    return () => window.removeEventListener("resize", place);
+  }, [commentAt, place]);
   const openComment = useMemo(
     () =>
       onComment
         ? {
-            open: (line: number) => {
+            open: (line: number, el?: HTMLElement) => {
+              anchorEl.current = el ?? null;
+              setAnchor(null);
+              place();
               setCommentAt(line);
               setUndelivered(false);
             },
             title: (line: number) => t("changes.commentLine", { n: line }),
           }
         : null,
-    [onComment, t]
+    [onComment, t, place]
   );
   const sendComment = () => {
     const text = draft.trim();
@@ -812,7 +832,10 @@ export function LaneDiffView({
           ref={root}
           data-testid="lane-diff-view"
           data-layout={layout}
-          onScroll={(e) => win.onScroll(e.currentTarget)}
+          onScroll={(e) => {
+            win.onScroll(e.currentTarget);
+            if (commentAt != null) place();
+          }}
           className={`flex flex-col font-mono text-[11px] leading-[1.45] ${
             layout === "split"
               ? boxHeight(fill, splitWindowed)
@@ -855,7 +878,19 @@ export function LaneDiffView({
         {commentAt != null && (
           <form
             data-testid="diff-comment-box"
-            className="flex flex-col gap-1 border-t border-border bg-surface-2 p-2"
+            style={
+              anchor
+                ? {
+                    position: "fixed",
+                    top: Math.min(anchor.top, window.innerHeight - 180),
+                    left: anchor.left,
+                    width: "min(28rem, calc(100vw - 2rem))",
+                  }
+                : undefined
+            }
+            className={`z-30 flex flex-col gap-1 border-border bg-surface-2 p-2 ${
+              anchor ? "rounded-lg border shadow-lg" : "border-t"
+            }`}
             onSubmit={(e) => {
               e.preventDefault();
               sendComment();