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>

relay/inc/bootstrap.php

@@ -232,6 +232,20 @@ $db->exec('CREATE TABLE IF NOT EXISTS collab_cards (
 )');
 $db->exec('CREATE INDEX IF NOT EXISTS idx_collab_cards_bucket_updated ON collab_cards(bucket_uid, updated_at)');
 
+// Collab attachments: binary assets referenced by collab cards (images,
+// PDFs, audio). Stored encrypted under the inter-tenant key. Scoped to
+// a bucket — all members of the bucket can fetch. Filename is the
+// hex-hash name that appears in card bodies as uploads/<name>.
+$db->exec('CREATE TABLE IF NOT EXISTS collab_attachments (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    bucket_uid TEXT NOT NULL,
+    filename TEXT NOT NULL,
+    data TEXT NOT NULL,
+    created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE (bucket_uid, filename)
+)');
+$db->exec('CREATE INDEX IF NOT EXISTS idx_collab_att_bucket ON collab_attachments(bucket_uid, created_at)');
+
 // Phase 4: advisory edit locks on collab cards. A client that opens the
 // editor for a collab card heartbeats into this table every 30s with a
 // 60s TTL. Other clients poll the list and show a lock indicator on the