fix(app): preview tabs persist across editor reload (restore open files on webview reload)

[jacktrnr]

Important

Problem — Reloading the editor (webview reload / Developer: Reload Window) clears the Preview tab: the .tex + .pdf you had open must be reopened by hand. The active file lives in the non-persisted ephemeral store (sessionPreviewFile), and the full open-tab set lives only in SessionPreviewTab's component-local workspace signal — both are wiped on reload, while sibling per-session UI state (open session tabs, reviewOpen, scroll) survives because it lives in the persisted serverGlobal store.
Approach — Persist the preview open-paths list (+ active file) in the per-session SessionView structure, reusing the Persist.serverGlobal("layout") machinery that already survives reload. Repoint previewFile from ephemeral → the persisted store as previewActive; add a persisted previewPaths list; wire SessionPreviewTab to restore its workspace from that list on mount and mirror open/close back up. This is the #1398 wiring, landed in the persisted store so it covers reload as well as remount.
Approaches Considered — (A) persisted SessionView store — chosen; smallest change, session-scoped for free, reuses the proven pattern. (B) VS Code webview getState/setState — a parallel persistence mechanism the app doesn't use for layout state; rejected. (C) opencode session.metadata (server DB) — stronger durability (survives full quit + fleet reconnection) but needs a server write per open/close; deferred as a follow-up on top of A.
Scope — in: persisting the flat list of open preview paths + the active path per session, so reload/remount restores every open preview tab; restoring only when the user hasn't already opened something this mount. out: persisting the split-pane tree layout / zoom (that's the full state codec, #981); full-quit / cross-machine durability (a C follow-up).
Assumptions — opencode.global.dat survives webview reload (proven: sessionTabs and sidebar width already persist through it). The Preview side-panel tab's open/active state already persists in sessionTabs, so the tab reopens on reload and its restored contents come from previewPaths.

Acceptance Criteria

• Open a .tex and its .pdf in the Preview tab, Developer: Reload Window — both preview tabs come back without reopening them, active tab preserved.
• Opening and closing preview files updates the persisted list (a closed file does not reappear after reload).
• A different Amicode session starts with its own preview paths, not the previous session's (no cross-session bleed).
• A persisted path whose file no longer exists renders the empty state on reload, not an error.
• Existing within-pane / tab-switch survival does not regress; the retained-pool lifecycle is untouched.

Verification recipe

1. Deploy: scripts/rebuild_amicode_locally.sh --mode local, then Developer: Reload Window once to pick up the binary.
2. Basic: open .tex + .pdf in Preview → plain Reload Window → both restore. (Pre-fix: gone.)
3. Session-scoped: open file A in session 1's Preview, switch to session 2, reload → session 2 shows its own; back to session 1 → A still there.
4. Stale path: open a file, delete it on disk, reload → empty state, no error.
5. Unit: bun test .../session-preview-tree.test.ts — the open-paths round-trip invariant (list restores in order, active = last, re-open idempotent, close shrinks the list).

Key Decisions

• Preview paths persist in the same serverGlobal SessionView store as reviewOpen/sessionTabs — not the webview state API, not a new store. Additive optional fields (previewActive, previewPaths); no schema-version bump (old data hydrates with them absent).
• SessionPreviewTab restores from the persisted list before the previewFile effect re-focuses the active file (openPreviewPath is idempotent), and the up-sync is gated on restore so an empty pre-hydration set never clobbers the persisted list.
• Only the flat path list + active path are persisted; split-pane layout is explicitly left to Add full Preview state codec for Work Column transfer #981.

Constraints & Invariants

• Must not regress the retained-pool lifecycle or the in-memory remount survival fix(app): preview open-paths lost on session re-instantiation (VS Code tab switch) #1398 covers.
• Keyed strictly per session — no cross-session preview bleed.
• Threading the open-paths props must not recreate/remount the retained-pool views.

Prior Art

The reviewFile / reviewOpen persisted setters and the Persist.serverGlobal target in layout.tsx; createPreviewWorkspace(paths) which already accepts an initial path list.

Source

Part of #1252. Sibling to #1398 (in-memory tab-switch survival — this implements its wiring but in the persisted store), #1365 (active-state plumbing), #981 (full split-pane state codec).