feat: add Tauri v2 Rust backend with PostgreSQL support

Add Rust backend: AppState with connection pool management,
TuskError handling, ConnectionConfig model, and all commands
for connections, schema browsing, query execution, data CRUD,
and CSV/JSON export via sqlx.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/commands/mod.rs

@@ -0,0 +1,5 @@
+pub mod connections;
+pub mod data;
+pub mod export;
+pub mod queries;
+pub mod schema;