Reconnect should restore previous pane and zoom state

[guysmoilov]

Problem

On reconnect, the client does not reliably return to the same pane/zoom context it was using before disconnect.

Requested behavior

When a client reconnects to the server with the same client identity/session context, it should try to restore:

1. The same tmux pane that was previously active
2. The same zoom state (zoomed or unzoomed)

Scope / Notes

• This should be best-effort: restore only if the target session/window/pane still exists and is accessible.
• If restoration is not possible, gracefully fall back to current default behavior.
• Avoid breaking initial-connect behavior for new clients.

Acceptance criteria

• After disconnect + reconnect, client lands on the same pane when that pane still exists.
• Zoom state is restored to match prior state when possible.
• If pane/session/window no longer exists, client reconnect still succeeds with sensible fallback.
• Behavior is covered by tests (at least integration-level for reconnect flow).

[guysmoilov]

Implementation research notes:

• Current reconnect behavior cannot preserve pane/zoom context because /ws/control always generates a fresh clientId on every auth and ignores auth.clientId from the client.
• Frontend currently does not persist/send a control clientId during control auth; it only uses the server-provided clientId to auth /ws/terminal.
• Control disconnect triggers cleanup that kills the per-client mobile grouped session (tmux-mobile-client-<clientId>). After reconnect, a new client/session is created, so prior active pane/zoom context is lost.
• There is no explicit zoom state in tmux snapshot types (TmuxWindowState) today, so restoration logic cannot directly compare desired/current zoom state.

Planned implementation (best-effort, with fallback):

1. Persist client identity on frontend (localStorage) and send clientId in control auth to allow server-side reconnect identity.
2. On backend control auth, reuse requested clientId when available and not already connected, so reconnects can map to prior client state.
3. Track per-client reconnect state (base session + active pane + active-window zoomed flag) from tmux snapshots.
4. On reconnect attach, prefer previously attached base session when it still exists; otherwise fall back to current picker/default behavior.
5. After attach, best-effort restore active pane and zoom state (only when target pane still exists and zoom state differs). Failures should not break reconnect.
6. Add integration tests covering reconnect identity reuse and pane/zoom restore behavior with graceful fallback.

[guysmoilov]

Implemented in worktree branch feat/issue-56-reconnect-pane-zoom.

What was changed:

• Reconnect identity reuse:
  • Frontend now persists clientId (localStorage) and sends it in control auth.
  • Backend control auth now accepts/reuses requested clientId (best-effort takeover of stale active context with same id).
• Reconnect state tracking:
  • Backend now tracks per-client reconnect state (baseSession, active paneId, active-window zoomed) from tmux state snapshots.
  • State is also updated on pane/zoom control mutations to reduce race windows.
• Restore on reconnect:
  • If prior base session still exists, reconnect prefers reattaching there (instead of forcing session picker).
  • After attach, backend attempts to restore previously active pane and zoom state.
  • Restore is best-effort: pane/zoom restore failures are logged and do not fail reconnect.
• Protocol/tmux snapshot support:
  • Added zoomed to TmuxWindowState (backend/frontend protocol types).
  • tmux window parsing now includes #{window_zoomed_flag}.
  • Fake tmux harness now models/toggles window zoom to support integration tests.

Tests added/updated:

• tests/integration/server.test.ts
  • reuses client identity and restores pane + zoom after reconnect
  • reconnect restore is best-effort when remembered pane no longer exists
• tests/backend/parser.test.ts updated for zoomed-window parsing.

Validation run in the worktree:

• npx vitest run tests/backend/parser.test.ts tests/integration/server.test.ts ✅
• npm run typecheck ✅
• npm test ✅ (smoke test remains skipped as before)