collab attachments, multi-day events, calendar improvements

- Collab buckets sync attachments alongside cards (images/PDFs/audio).
  New /api/collab-attachments endpoint on relay, encrypted at rest,
  bucket-scoped for members.
- CalDAV multi-day events via **Bis:** body field. Calendar renders
  event on every day in range with day counter (1/6, 2/6, ...).
- Calendar preserves month across close/reopen (only jumps to today
  on first open).
- Calendar title: centered nav group with [MM-DD] Mon Y [KW-KW] —
  dimmed brackets, fixed width so nothing wiggles.
- Editor: scratchpad uses custom confirm, autobackup-draft dialog
  replaced with custom styled dialog.
- Scanlines theme feature removed (performance issue).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/tests/calendar.test.js

@@ -58,4 +58,25 @@ describe('Calendar', () => {
         await sleep(150);
         expect($('#calendar')).toBeHidden();
     });
+
+    it('preserves month across close/reopen', async () => {
+        // Open calendar and navigate forward 3 months
+        click('#calendar-btn');
+        await sleep(200);
+        click('#cal-next');
+        click('#cal-next');
+        click('#cal-next');
+        await sleep(200);
+        const navigatedTitle = $('#cal-title').textContent;
+
+        // Close
+        click('#cal-close');
+        await sleep(150);
+        expect($('#calendar')).toBeHidden();
+
+        // Reopen — must stay on the navigated month, not jump to today
+        click('#calendar-btn');
+        await sleep(200);
+        expect($('#cal-title').textContent).toBe(navigatedTitle);
+    });
 });