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/renderer/src/global.d.ts

@@ -28,17 +28,31 @@ export type CaptureResult =
   | { ok: false; reason: 'unconfigured' | 'error'; message?: string }
   | ({ ok: true } & CaptureProposal)
 
+/** A reconciled snapshot as it crosses the bridge. */
+export interface SnapshotPayload {
+  configured: boolean
+  issues: GiteaIssue[]
+  milestones: GiteaMilestone[]
+  deps: DependencyEdge[]
+  /** Normalized lifecycle events keyed by issue number. */
+  timelines: Record<number, LifecycleEvent[]>
+  /** true when served from the persisted cache (offline / instant boot). */
+  stale?: boolean
+  /** ISO time the persisted snapshot was reconciled (present on cached reads). */
+  savedAt?: string
+}
+
+/** Boot payload — the persisted snapshot, or a marker that there's none yet. */
+export type BootPayload =
+  | { configured: false }
+  | { configured: true; cached: false }
+  | ({ configured: true; cached: true } & Omit<SnapshotPayload, 'configured'>)
+
 /** The gitea bridge exposed by the preload over IPC (main-process backed). */
 export interface GiteaBridge {
   status(): Promise<{ configured: boolean; repo: string | null }>
-  reconcile(): Promise<{
-    configured: boolean
-    issues: GiteaIssue[]
-    milestones: GiteaMilestone[]
-    deps: DependencyEdge[]
-    /** Normalized lifecycle events keyed by issue number. */
-    timelines: Record<number, LifecycleEvent[]>
-  }>
+  boot(): Promise<BootPayload>
+  reconcile(): Promise<SnapshotPayload>
   getIssue(index: number): Promise<GiteaIssue | null>
   applyChange(change: IssueChange): Promise<ApplyChangeResult>
   createIssues(issues: ProposedIssue[]): Promise<CreateIssuesResult>

apps/desktop/src/renderer/src/lib/use-backlog.ts

@@ -12,13 +12,18 @@ export type BacklogState =
       milestones: GiteaMilestone[]
       deps: DependencyEdge[]
       timelines: Record<number, LifecycleEvent[]>
+      /** true while showing the persisted snapshot (instant boot / offline). */
+      stale: boolean
+      /** ISO time the shown snapshot was reconciled, when stale. */
+      savedAt?: string
     }
 
 /**
- * Reconcile the managed repo through the main-process bridge. Runs once on
- * mount; the returned `refetch` re-reconciles after a write so the board and
- * forecast reflect the change. `unconfigured` means no token — the UI falls
- * back to demo fixtures. Errors (network, bad token) surface as `error`.
+ * Reconcile the managed repo through the main-process bridge, stale-while-
+ * revalidate: on mount it shows the persisted snapshot instantly (marked stale),
+ * then a fresh reconcile supersedes it. If gitea is unreachable, the fresh
+ * reconcile falls back to the persisted snapshot (offline). `refetch` re-syncs
+ * after a write. `unconfigured` means no token — the UI uses demo fixtures.
  */
 export function useBacklog(): [BacklogState, () => void] {
   const [state, setState] = useState<BacklogState>({ status: 'loading' })
@@ -27,6 +32,30 @@ export function useBacklog(): [BacklogState, () => void] {
 
   useEffect(() => {
     let alive = true
+
+    // instant boot from the persisted snapshot (only on first mount, not refetch)
+    if (nonce === 0) {
+      window.commitea.gitea
+        .boot()
+        .then((b) => {
+          if (!alive || !('cached' in b) || !b.cached) return
+          setState((prev) =>
+            prev.status === 'ready' && !prev.stale
+              ? prev // a fresh reconcile already won the race
+              : {
+                  status: 'ready',
+                  issues: b.issues,
+                  milestones: b.milestones,
+                  deps: b.deps,
+                  timelines: b.timelines,
+                  stale: true,
+                  savedAt: b.savedAt,
+                },
+          )
+        })
+        .catch(() => {})
+    }
+
     window.commitea.gitea
       .reconcile()
       .then((r) => {
@@ -39,12 +68,19 @@ export function useBacklog(): [BacklogState, () => void] {
                 milestones: r.milestones,
                 deps: r.deps,
                 timelines: r.timelines,
+                stale: r.stale ?? false,
+                savedAt: r.savedAt,
               }
             : { status: 'unconfigured' },
         )
       })
       .catch((e: unknown) => {
-        if (alive) setState({ status: 'error', message: e instanceof Error ? e.message : String(e) })
+        if (alive) {
+          // keep a shown boot snapshot rather than clobbering it with an error
+          setState((prev) =>
+            prev.status === 'ready' ? prev : { status: 'error', message: e instanceof Error ? e.message : String(e) },
+          )
+        }
       })
     return () => {
       alive = false