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/css/style.css

@@ -29,7 +29,6 @@ body { font-family: var(--t-font-content, 'JetBrains Mono', monospace); }
 }
 .font-pixel { font-family: var(--t-font-ui, var(--t-font-pixel, 'Silkscreen', cursive)) !important; }
 .brand { font-family: 'Silkscreen', cursive !important; }
-body.no-scanlines::after { display: none !important; }
 #dialog-msg { white-space: pre-line; word-break: break-word; }
 
 /* ── User-customizable design tokens ── */
@@ -172,7 +171,6 @@ body.anim-slow .card-spinner {
     position: relative; padding: 14px; overflow: hidden;
     background: var(--t-bg); border: 1px solid var(--t-border); border-radius: 3px;
 }
-.theme-preview .tp-scan { position: absolute; inset: 0; pointer-events: none; background: repeating-linear-gradient(0deg, transparent, transparent 2px, var(--t-scanline, rgba(0,0,0,0.04)) 2px, var(--t-scanline, rgba(0,0,0,0.04)) 4px); }
 .theme-preview .tp-row { display: flex; gap: 12px; align-items: stretch; position: relative; z-index: 1; }
 .theme-preview .tp-card {
     flex: 1; min-width: 0;
@@ -299,16 +297,6 @@ body.settings-peeking #settings-overlay > div { pointer-events: auto; }
 *::-webkit-scrollbar-thumb:hover { background: var(--t-dim); }
 *::-webkit-scrollbar-corner { background: transparent; }
 
-/* Scanlines */
-body::after {
-    content: '';
-    position: fixed;
-    inset: 0;
-    pointer-events: none;
-    z-index: 9999;
-    background: repeating-linear-gradient(0deg, transparent, transparent 2px, var(--t-scanline, rgba(0,0,0,0.04)) 2px, var(--t-scanline, rgba(0,0,0,0.04)) 4px);
-}
-
 /* Blinking cursor */
 .cursor-blink::after { content: '_'; animation: blink 1s step-end infinite; }
 @keyframes blink { 50% { opacity: 0; } }