feat(themes): custom CSS themes per user (upload, paste, live)

- Theme manager in settings: upload a .css file or paste CSS, template
  download/copy/insert, edit and delete; custom themes show up in the
  gallery and the quick switchers
- Themes are injected live as <style id="mindsift-custom-theme">; the
  active one is cached so it applies on boot without flashing
- Backend: user_themes table and GET/POST /themes, DELETE /themes/{id}
  (per user, max 200 KB)
- Docs: theming, REST, database

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/sqlite.go

@@ -368,6 +368,18 @@ func (s *Store) migrate() error {
 
 	CREATE INDEX IF NOT EXISTS idx_revoked_tokens_expires ON revoked_tokens(expires_at);
 	CREATE INDEX IF NOT EXISTS idx_revoked_tokens_user ON revoked_tokens(user_id);
+
+	-- Custom CSS themes uploaded or pasted by a user (stored in the volume, survive updates)
+	CREATE TABLE IF NOT EXISTS user_themes (
+		id TEXT PRIMARY KEY,
+		user_id TEXT NOT NULL,
+		name TEXT NOT NULL,
+		css_content TEXT NOT NULL,
+		created_at TEXT NOT NULL,
+		updated_at TEXT NOT NULL,
+		FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE
+	);
+	CREATE INDEX IF NOT EXISTS idx_user_themes_user ON user_themes(user_id);
 	`
 	if _, err := s.db.Exec(schema); err != nil {
 		return err