feat: persist the reconcile cache to disk — instant boot + offline reads

Extends the in-memory cache into a durable mirror. The reconcile snapshot is
written to disk on every successful reconcile; on boot the app shows it instantly
(stale-while-revalidate) instead of a blank board, and if gitea is unreachable,
reads fall back to it (offline). Rebuildable — the durable truth stays in gitea.

- snapshot-store.ts: load/save the snapshot as JSON in app userData (never throws;
  corrupt/absent → "no cache"). At this scale (~34 issues, 37KB) the whole snapshot
  fits in memory, so a JSON file beats indexed SQL — no query benefit yet, no
  native-module (better-sqlite3/electron-rebuild) or WASM dependency. That's the
  next step if the mirror ever needs indexed queries over larger data.
- gitea.ts: getSnapshot persists on a fresh pull; bootSnapshot() returns the
  persisted snapshot (without seeding the cache — agents still reconcile fresh);
  gitea:boot serves it; gitea:reconcile falls back to it on failure (stale:true).
- useBacklog: stale-while-revalidate — boot instantly, then a fresh reconcile
  supersedes; a reconcile error keeps the shown snapshot instead of erroring.

Verified: desktop typecheck clean, 14 fixture e2e green. Live: the snapshot
persists (34 issues / 44 deps / 34 timelines / 5 milestones written to disk); a
second launch with gitea unreachable renders the full real board — NOW/NEXT/LATER
+ the Monte Carlo cone — entirely from the cache (new live-persistence e2e).

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

apps/desktop/src/preload/index.ts

@@ -5,6 +5,8 @@ const api = {
   gitea: {
     /** Whether the main process has a gitea token + target repo configured. */
     status: () => ipcRenderer.invoke('gitea:status'),
+    /** The last persisted snapshot, for instant boot before the fresh reconcile. */
+    boot: () => ipcRenderer.invoke('gitea:boot'),
     /** Full read of the managed repo — every issue + milestone. */
     reconcile: () => ipcRenderer.invoke('gitea:reconcile'),
     /** One issue by index, normalized (or null if unconfigured). */