feat: add Turso/libSQL database support

Turso is a distributed SQLite database built on libSQL, offering HTTP-based
connectivity with auth token authentication. This change adds full support:

- New DatabaseType::Turso and PoolKind::Turso variants
- TursoClient driver using libSQL HTTP pipeline API (/v2/pipeline)
- Bearer token auth via password or url_params (auth_token=)
- Multi-statement batch pipeline for transactional integrity
- Standalone BEGIN/COMMIT/ROLLBACK treated as no-ops (pipeline is auto-commit)
- Full metadata support: tables, columns, indexes, foreign keys, triggers, DDL
- Frontend: connection form, SQL completion (SQLite syntax), capability sets
- Built-in databases (SQLite/Turso/DuckDB/RQLite/Access) placed first in selector
- Unit tests (15) + integration tests against live libsql-server (12)
- Single connection pool, skips TCP probe

Closes #948

apps/desktop/src/components/objects/ObjectBrowser.vue

@@ -198,6 +198,7 @@ const sourceFormatDialect = computed<SqlFormatDialect>(() => {
     case "sqlserver":
       return effectiveDatabaseType.value;
     case "rqlite":
+    case "turso":
       return "sqlite";
     case "gaussdb":
     case "kwdb":