fix(frontend): refit web agent cells after opening a new one (#61 web regression)

The web shell never adopted the desktop's #61 fix: WebAgentCell/WebWorkspace
didn't pass any refitSignal to TerminalView, so a cell opened after another
kept a stale xterm scaling — desktop "fixed" it via an incidental window
resize, but mobile has no such escape hatch.

- TerminalView: a refit landing on a transient 0x0 container now reschedules
  on the next few frames instead of giving up for good (bounded retries),
  closing the independent timing gap Architect identified. refitSignal stays
  the single explicit-refit mechanism; the terminal/PTY is never recreated,
  and resize is still pushed to the PTY only when rows/cols actually change.
- WebAgentCell: new optional refitSignal prop, forwarded to TerminalView
  as-is (no key change, no remount).
- WebWorkspace/LiveProjectPanel: new cellLayoutVersion counter, the web
  equivalent of desktop useLayout.layoutVersion, bumped on every cell
  open/close and forwarded as refitSignal to the visible WebAgentCell.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/web/WebAgentCell.tsx

@@ -39,9 +39,16 @@ interface WebAgentCellProps {
   cwd: string;
   /** Layout leaf id, when the caller tracks one (drives the singleton guard). */
   nodeId?: string;
+  /**
+   * Bumped by the caller after a web cell opens/closes (ticket #61 web
+   * regression) — the web equivalent of desktop `useLayout.layoutVersion`.
+   * Forwarded to {@link TerminalView} as-is; never changes this component's
+   * `key`, so the terminal/PTY is never recreated by it.
+   */
+  refitSignal?: number;
 }
 
-export function WebAgentCell({ projectId, agentId, cwd, nodeId }: WebAgentCellProps) {
+export function WebAgentCell({ projectId, agentId, cwd, nodeId, refitSignal }: WebAgentCellProps) {
   const { agent } = useGateways();
   const [sessionId, setSessionId] = useState<string | null>(null);
   // Input API of the mounted xterm (#69), used by the mobile key bar. Stays
@@ -85,6 +92,7 @@ export function WebAgentCell({ projectId, agentId, cwd, nodeId }: WebAgentCellPr
           sessionId={sessionId}
           onSessionId={setSessionId}
           onReady={setInputApi}
+          refitSignal={refitSignal}
         />
       </div>
       <TerminalKeyBar api={inputApi} />