Add ICS import/export with course linking, QR sync improvements, and UI polish

- Group imported ICS occurrences per weekly slot, hatch irregular/half-semester
  sessions, and link them to existing courses by stable code (not name, since
  university exports mix languages) with a one-time manual-match fallback
- Preserve exact per-date occurrences from imported ICS so exported calendars
  correctly skip holiday/vacation gaps instead of assuming a flat weekly rule
- Add "Export ICS" for a semester's course + manual schedule, prompting once
  for semester start/end dates (now also editable directly on the semester)
- Fix pinned QRCode CDN version (package dropped its browser bundle) and add
  a send/receive choice after scanning instead of always auto two-way syncing
- Fix course save silently adding a phantom Monday slot from picker defaults;
  replace with an explicit "No fixed schedule / Has a weekly schedule" toggle
- Service worker: network-first app shell so edits show up without manual
  cache-busting, self-update + reload on new worker, version indicator
- Dashboard: fold older semesters by default and jump to the current one,
  optional auto-fold for validated blocks/sub-blocks, JSON backup nudge

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sw.js

@@ -1,48 +1,72 @@
-const CACHE_NAME = 'credittracker-v36';
-const ASSETS = [
-  './index.html',
-  './manifest.json',
-  './icon.svg',
-  './icon-maskable.svg'
-];
-
-// Install: pre-cache all app shell files
-self.addEventListener('install', event => {
-  event.waitUntil(
-    caches.open(CACHE_NAME).then(cache => cache.addAll(ASSETS))
-  );
-  self.skipWaiting();
-});
-
-// Activate: clean up old caches
-self.addEventListener('activate', event => {
-  event.waitUntil(
-    caches.keys().then(keys =>
-      Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k)))
-    )
-  );
-  self.clients.claim();
-});
-
-// Fetch: cache-first for app files, network-first for everything else
-self.addEventListener('fetch', event => {
-  // Only handle same-origin GET requests
-  if (event.request.method !== 'GET') return;
-
-  event.respondWith(
-    caches.match(event.request).then(cached => {
-      if (cached) return cached;
-      return fetch(event.request).then(response => {
-        // Cache successful responses for app assets
-        if (response.ok) {
-          const clone = response.clone();
-          caches.open(CACHE_NAME).then(cache => cache.put(event.request, clone));
-        }
-        return response;
-      });
-    }).catch(() => {
-      // Offline fallback: return the app shell
-      return caches.match('./index.html');
-    })
-  );
-});
+const CACHE_NAME = 'credittracker-v47';
+const ASSETS = [
+  './index.html',
+  './manifest.json',
+  './icon.svg',
+  './icon-maskable.svg'
+];
+
+// Install: pre-cache all app shell files
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then(cache => cache.addAll(ASSETS))
+  );
+  self.skipWaiting();
+});
+
+// Activate: clean up old caches
+self.addEventListener('activate', event => {
+  event.waitUntil(
+    caches.keys().then(keys =>
+      Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+// Lets the page ask "what version are you actually running" (Settings screen indicator)
+self.addEventListener('message', event => {
+  if (event.data && event.data.type === 'GET_VERSION') {
+    event.source.postMessage({ type: 'VERSION', version: CACHE_NAME });
+  }
+});
+
+// Fetch: network-first for the app shell (so edits show up without a version bump),
+// cache-first for everything else (icons, manifest, external libs)
+self.addEventListener('fetch', event => {
+  if (event.request.method !== 'GET') return;
+
+  const reqUrl = new URL(event.request.url);
+  const isSameOrigin = reqUrl.origin === self.location.origin;
+  const isAppShell = isSameOrigin && (event.request.mode === 'navigate' || reqUrl.pathname.endsWith('/index.html'));
+
+  if (isAppShell) {
+    event.respondWith(
+      fetch(event.request).then(response => {
+        if (response.ok) {
+          const clone = response.clone();
+          caches.open(CACHE_NAME).then(cache => cache.put(event.request, clone));
+        }
+        return response;
+      }).catch(() => caches.match(event.request).then(cached => cached || caches.match('./index.html')))
+    );
+    return;
+  }
+
+  event.respondWith(
+    caches.match(event.request).then(cached => {
+      if (cached) return cached;
+      return fetch(event.request).then(response => {
+        if (response.ok && isSameOrigin) {
+          const clone = response.clone();
+          caches.open(CACHE_NAME).then(cache => cache.put(event.request, clone));
+        }
+        return response;
+      });
+    }).catch(() => {
+      // Offline fallback only for external CDN fetches: fail loudly instead of masking with app HTML
+      if (isSameOrigin) return caches.match('./index.html');
+      return Response.error();
+    })
+  );
+});