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/AddLaneModal.tsx

@@ -65,7 +65,7 @@ function Seg({
       aria-pressed={active}
       className={`flex-1 rounded px-2 py-1 text-xs font-medium transition-colors ${
         active
-          ? "bg-accent text-white shadow-sm"
+          ? "bg-accent-solid text-white shadow-sm"
           : "text-fg-secondary hover:bg-surface-3 hover:text-fg-primary"
       }`}
     >
@@ -567,7 +567,7 @@ export function AddLaneModal({
                       id="add-lane-base"
                       value={base}
                       onChange={(e) => setBase(e.target.value)}
-                      className="w-full rounded-md border border-border-light bg-surface-0 px-3 py-1.5 text-xs text-fg-primary focus:border-accent focus:outline-none"
+                      className="select w-full !py-1.5 !text-xs"
                     >
                       {branches.map((b) => (
                         <option key={b} value={b}>
@@ -606,7 +606,7 @@ export function AddLaneModal({
                     id="add-lane-existing"
                     value={branch}
                     onChange={(e) => setBranch(e.target.value)}
-                    className="w-full rounded-md border border-border-light bg-surface-0 px-3 py-1.5 font-mono text-xs text-fg-primary focus:border-accent focus:outline-none"
+                    className="select w-full !py-1.5 font-mono !text-xs"
                   >
                     {branches.map((b) => {
                       const busyAt = busyBranches.get(b);