Messages: Add archive (soft-delete) support + dropdown UI polish

- Implemented `/account/messages/archive` route for soft-archiving messages
- Added `is_archived` flag to `users_messages` schema and model
- Topbar dropdown now reflects accurate unread message count
- Fixed missing route registration for archive handler
- Improved message visibility checks to prevent access violations
- Placeholder for rate-limit (429) error page rendering identified

storage/schema.go

@@ -119,6 +119,7 @@ CREATE TABLE IF NOT EXISTS users_messages (
 	subject TEXT NOT NULL,
 	message TEXT,
 	is_read BOOLEAN DEFAULT FALSE,
+	is_archived BOOLEAN DEFAULT FALSE,
 	created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
 );`