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/routes/collab-cards.php

@@ -199,3 +199,66 @@ if (preg_match('#^/api/collab-cards/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$#', $route
     ]);
     jsonResponse(['ok' => true]);
 }
+
+// ── Collab attachments (images/PDFs/audio referenced by collab cards) ─
+
+if ($route === '/api/collab-attachments' && $method === 'POST') {
+    if (!$clientTenant) jsonResponse(['error' => 'unauthorized'], 401);
+    $input = json_decode(file_get_contents('php://input'), true) ?: [];
+    $bucketUid = (string)($input['bucket_uid'] ?? '');
+    $filename = (string)($input['filename'] ?? '');
+    $data = (string)($input['data'] ?? '');
+    if (!$bucketUid || !$filename || !$data) jsonResponse(['error' => 'bucket_uid, filename, and data required'], 400);
+    if (!collabCardsIsMember($db, $bucketUid, (int)$clientTenant['id'])) {
+        jsonResponse(['error' => 'not a member of this bucket'], 403);
+    }
+
+    // Decrypt-reencrypt hop: data arrives encrypted with sender's secret,
+    // stored at rest under the inter-tenant key.
+    if (empty($clientTenant['secret_encrypted'])) jsonResponse(['error' => 'sender has no client secret'], 400);
+    $senderSecret = masterDecrypt($clientTenant['secret_encrypted'], $MASTER_KEY);
+    if (!$senderSecret) jsonResponse(['error' => 'sender secret unavailable'], 500);
+    $interKey = getInterTenantKey($db, $MASTER_KEY);
+    if (!$interKey) jsonResponse(['error' => 'relay super-shared key not set'], 500);
+
+    $plain = clientDecrypt($data, $senderSecret);
+    if ($plain === '') jsonResponse(['error' => 'attachment decrypt failed'], 400);
+    $stored = clientEncrypt($plain, $interKey);
+
+    $db->prepare('INSERT INTO collab_attachments (bucket_uid, filename, data) VALUES (:b, :f, :d)
+        ON CONFLICT(bucket_uid, filename) DO UPDATE SET data = excluded.data')
+        ->execute(['b' => $bucketUid, 'f' => $filename, 'd' => $stored]);
+    jsonResponse(['ok' => true]);
+}
+
+if ($route === '/api/collab-attachments' && $method === 'GET') {
+    if (!$clientTenant) jsonResponse(['error' => 'unauthorized'], 401);
+    $bucketUid = trim($_GET['bucket_uid'] ?? '');
+    $filename = trim($_GET['filename'] ?? '');
+    if (!$bucketUid) jsonResponse(['error' => 'bucket_uid required'], 400);
+    if (!collabCardsIsMember($db, $bucketUid, (int)$clientTenant['id'])) {
+        jsonResponse(['error' => 'not a member of this bucket'], 403);
+    }
+    if (empty($clientTenant['secret_encrypted'])) jsonResponse(['error' => 'no client secret set'], 400);
+    $recipientSecret = masterDecrypt($clientTenant['secret_encrypted'], $MASTER_KEY);
+    if (!$recipientSecret) jsonResponse(['error' => 'recipient secret unavailable'], 500);
+    $interKey = getInterTenantKey($db, $MASTER_KEY);
+    if (!$interKey) jsonResponse(['error' => 'relay super-shared key not set'], 500);
+
+    if ($filename) {
+        $stmt = $db->prepare('SELECT filename, data FROM collab_attachments WHERE bucket_uid = :b AND filename = :f');
+        $stmt->execute(['b' => $bucketUid, 'f' => $filename]);
+    } else {
+        // List all filenames for this bucket (no data — client picks what to fetch)
+        $stmt = $db->prepare('SELECT filename FROM collab_attachments WHERE bucket_uid = :b ORDER BY created_at DESC');
+        $stmt->execute(['b' => $bucketUid]);
+        jsonResponse(['ok' => true, 'files' => array_column($stmt->fetchAll(), 'filename')]);
+    }
+    $rows = $stmt->fetchAll();
+    $out = [];
+    foreach ($rows as $row) {
+        $plain = clientDecrypt($row['data'] ?? '', $interKey);
+        $out[] = ['filename' => $row['filename'], 'data' => clientEncrypt($plain, $recipientSecret)];
+    }
+    jsonResponse(['ok' => true, 'attachments' => $out]);
+}