fix(desktop): session tabs no longer trap — self-heal phantom tiles, never hide chat tabs behind a hidden strip

[leonphull]

Summary

Fixes two production-observed traps that make desktop chat tabs appear completely broken after a crash or an unlucky toggle. Both were hit today on macOS (remote/SSH connection mode) while recovering from the FD-exhaustion outage (#78873); root-caused live via CDP against the affected machines.

Fixes #79476, fixes #79518.

1. Phantom session tiles (crash-stale storage) — #79476

Symptom: "Open in new tab" vanishes from the session context menu; Cmd-click on sidebar sessions is a dead click.

Root cause: $sessionTiles (localStorage) and the layout tree persist separately. A hard restart can commit one without the other; focusOpenSession() trusted the tile list unconditionally and returned 'tile' for a pane that doesn't exist. Downstream, openSession(..., 'tab') early-returns (dead click) and the actions menu computes alreadyTabbed=true forever.

Fix: after the reveal attempt, if the pane is still absent from the layout tree, the persisted tile is phantom — discardSessionTile() it and report a miss so the caller opens the session for real. Self-heals on first interaction instead of requiring a manual localStorage wipe.

In-flight guard: tiles opened during the current run are exempt (pane adoption is async via the registry; a just-opened tile is briefly pane-less and must not be discarded mid-flight). Only storage-hydrated tiles can be judged phantom.

2. Hidden tab strip dead end — #79518

Symptom: every sidebar click on an open session looks dead; new tabs arrive invisible; there is NO way back (the only "Show header" affordance lives in the strip's own context menu).

Root cause: two sites preserve headerHidden where the pane is a chat tab: revealTreePane() (fronts session tabs into an invisible strip) and the adoption path (re-pins the zone's hidden flag onto newly adopted session tiles — the "standing preference" branch, correct for tool panels only).

Fix: use the existing isSessionStripPane() distinction at both sites. Revealing or adopting a session strip pane (workspace / session-tile:*) into a header-hidden zone force-shows the bar; tool panels keep honoring the standing preference (terminal close/reopen keeps working as designed).

This follows the desktop AGENTS.md doctrine: recovery ladders must end in a real affordance, and persisted UI state is a cache to validate at the boundary, not owned truth.

Tests

• src/store/session-states.test.ts: phantom tile dropped + reported as miss; in-flight tile survives the adoption gap (simulates the exact mid-write crash shape).
• src/components/pane-shell/tree/hidden-strip-recovery.test.ts (new): reveal un-hides for session tiles; adoption un-hides for new session tiles; tool panels still honor the hidden preference.
• Full desktop suite: 4418 passed | 2 skipped on this branch.

Verification beyond unit tests

Both fixes were validated against the live production instances that hit the bugs (CDP Runtime.evaluate against the packaged app): the phantom-tile state and the hidden-strip state were reproduced from the users' real localStorage payloads, and the patched build restores tab behavior on first interaction.

[matebenyovszky]

Cross-referencing for whoever reviews this area: there's a third zone class with the same "trapped behind a hidden strip" shape — a preview / Browser tile docked in its own zone. Once its header is double-tap-hidden it has no tab, no ✕ and no zone menu, and neither the session self-heal here nor the sidebar-row restore in #81638 reaches it (no session state, no sidebar row).

Opened #84458 for that one: a top-edge reveal strip on the zone itself, so the way back doesn't depend on another surface existing.

Not suggesting any change to this PR — just so the three don't get reviewed as duplicates of each other.