fix(tabs): preserve label across transient remote-host disconnects

[attson]

Repro

Open several remote-mirrored tabs from a single desktop host. Let the relay poll for a while; when that host's network/process briefly drops (or it ever does), the relay's session list stops including those IDs for one or more polls — and every tab that referenced them collapses to "(空)" / "(empty)". Even after the host comes back, the title doesn't.

Root cause

`sweepMissingSessions()` reacts to the relay's new session list by nulling `pane.sessionId` for every pane whose session isn't there anymore. After that, the TabBar's `shortTitle()` sees `activeSession === null` and renders `i18n("terminal.emptyTab")` — i.e. "(空)". The tab is now indistinguishable from any other empty pane.

Fix

• `Pane` gains an optional `lastSeenInfo: SessionInfo` field.
• Both `applyLocalSessions` and `applyRemoteSessions` snapshot the currently-known sessions (local + remote) before replacing the list, and pass that snapshot to `sweepMissingSessions`.
• When sweep nulls a pane it stashes the snapshot's entry for that id (or the pane's existing cache if two consecutive sweeps fire) onto `lastSeenInfo`.
• `tabSummaries` falls back to `lastSeenInfo` and surfaces a `disconnected: true` flag.
• `TabBar` uses the fallback for the title, dims + italicises the title, and the remote dot turns grey. Tooltip prefixes "(disconnected)" via a new i18n key.

If the same `session_id` reappears in a later poll (transient blip resolved), `findSessionInfo` returns the live entry and the disconnected styling is gone automatically — no manual reattach needed. If the host returns with new session ids, the cached label still tells the user which tab is which.

Test plan

• New vitest case: TabBar renders the cached cwd basename + "disconnected" class when `disconnected: true` is passed
• `npm test` — 77 files / 645 tests green
• `npx vue-tsc --noEmit` — clean
• manual: open ≥2 remote tabs, kill the host process / disable wifi briefly. Tabs keep their titles instead of "(空)" and visibly grey out. Restart host with same session: tab returns to normal style.