fixes: scratchpad, zen ESC, chat time, tooltip safety, collab sync race

- Scratchpad: card-scoped private notes in editor (toggle vs preview),
  smaller font, cleared via button, stored in cards.scratchpad
- Zen mode: ESC exits zen before closing detail, Z toggles zen
- Chat: always show absolute time (hh:mm, yesterday, or date+time)
- Backspace/Delete also deletes focused card (not just selected)
- Selected + focused cards use brand accent (not accent2)
- Tooltip: guard against non-element targets
- Collab sync: skip poll while editor is open, 2s push debounce
- Rrule test: only check geburtstag tag, not title format

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

src/Database.php

@@ -152,6 +152,10 @@ class Database
             self::$pdo->exec('CREATE INDEX IF NOT EXISTS idx_buckets_parent_id ON buckets(parent_id)');
         }
 
+        if (!in_array('scratchpad', $colNames)) {
+            self::$pdo->exec("ALTER TABLE cards ADD COLUMN scratchpad TEXT DEFAULT NULL");
+        }
+
         // Card edit history
         self::$pdo->exec('
             CREATE TABLE IF NOT EXISTS card_history (