feat(grid): v3a — read-only monitor quadrants (no shared-PTY shrink)

A quadrant can be flipped to "monitor" mode via a 👁 header toggle: instead of
the live interactive terminal, it renders the session's screen from periodic
GET /live-sessions/:id/preview snapshots into a read-only xterm. A monitored
quadrant never attaches a WS and never sends a resize, so — unlike a live
quadrant (latest-writer-wins) — it does not drive the shared PTY size. That lets
you watch a session in a small quadrant without shrinking it for another device
using it full-screen (the cross-device shrink the split-grid design flagged).

- public/cell-monitor.ts (new): mountCellMonitor(host, id) — polling read-only
  preview, scaled to fit, best-effort, disposes cleanly (no write after dispose).
- tabs.ts: per-entry monitor state; applyLayout keeps the live pane hidden and
  starts/stops the monitor; 👁 toggle in the cell header; teardown on close /
  leaving grid mode.
- style.css: monitor button + .cell-monitor container.
- tests: cell-monitor.test.ts (poll→write, dispose, late-resolve guard) + monitor
  wiring in tabs.test.ts. typecheck + build clean, 1587 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vitest.config.ts

@@ -25,6 +25,7 @@ export default defineConfig({
         'public/terminal-session.ts',
         'public/tabs.ts',
         'public/grid-layout.ts',
+        'public/cell-monitor.ts',
         'public/preview-grid.ts',
         'public/title-util.ts',
         'public/voice-commands.ts',