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>

.claude

@@ -1,17 +1,17 @@
-I need your help for a small web app i'd like to make.
-
-I take a lot of courses and i need to plannify when to take them and how much they give.
-
-Each course has this attributes:
-- Study plan
-    - Block
-- name
-- credits
-- semester (Autumn or Spring)
-- Grade (if >= 4 or block.mean() >= 4, passed, else, failed)
-
-I'd like to have it as a small webapp i can use on my phone, where i can easily manage my different semester (being able to create one), add my grade, manage my study plan (able to add course to a new plan, an existing one or without study plan), choose the block the course is in (if in a study plan)
-
-Everything must be sumerized into tables (global, per study plan, per semester)
-
+I need your help for a small web app i'd like to make.
+
+I take a lot of courses and i need to plannify when to take them and how much they give.
+
+Each course has this attributes:
+- Study plan
+    - Block
+- name
+- credits
+- semester (Autumn or Spring)
+- Grade (if >= 4 or block.mean() >= 4, passed, else, failed)
+
+I'd like to have it as a small webapp i can use on my phone, where i can easily manage my different semester (being able to create one), add my grade, manage my study plan (able to add course to a new plan, an existing one or without study plan), choose the block the course is in (if in a study plan)
+
+Everything must be sumerized into tables (global, per study plan, per semester)
+
 Can you help me?