Offline Queue: IndexedDB PhotoQueue, offline routing, SW caching + sync

- PhotoQueue class with IndexedDB store (queued/uploading/done/failed)
- uploadSelected() and startBulkProcess() check navigator.onLine and queue offline
- Online event auto-drains queue via /api/bulk-process with exponential backoff
- Offline banner shows queued count, hides when empty
- Service Worker: cache-first static, network-first API, background sync listener
- Exponential backoff: 2s → 5s → 15s, then mark failed after 3 retries

static/sw.js

@@ -0,0 +1,49 @@
+// EXIF Scanner — Service Worker
+// Cache-first for static assets, network-first for API.
+const CACHE = "exif-cache-v1";
+const STATIC = ["/", "/static/"];
+
+self.addEventListener("install", (e) => {
+  self.skipWaiting();
+  e.waitUntil(
+    caches.open(CACHE).then((c) => c.addAll(["/", "/static/index.html"]))
+  );
+});
+
+self.addEventListener("activate", (e) => {
+  e.waitUntil(clients.claim());
+});
+
+self.addEventListener("fetch", (e) => {
+  const url = new URL(e.request.url);
+  // API calls: network-first
+  if (url.pathname.startsWith("/api/")) {
+    e.respondWith(
+      fetch(e.request).catch(() => {
+        return new Response(
+          JSON.stringify({ offline: true, error: "No network connection" }),
+          { status: 503, headers: { "Content-Type": "application/json" } }
+        );
+      })
+    );
+    return;
+  }
+  // Static assets: cache-first
+  e.respondWith(
+    caches.match(e.request).then((r) => r || fetch(e.request))
+  );
+});
+
+// Background Sync
+self.addEventListener("sync", (e) => {
+  if (e.tag === "sync-photos") {
+    e.waitUntil(syncPhotos());
+  }
+});
+
+async function syncPhotos() {
+  const clients = await self.clients.matchAll();
+  for (const client of clients) {
+    client.postMessage({ type: "SYNC_TRIGGERED" });
+  }
+}