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/lib.rs

@@ -0,0 +1,46 @@
+mod commands;
+mod error;
+mod models;
+mod state;
+
+use state::AppState;
+
+pub fn run() {
+    tauri::Builder::default()
+        .plugin(tauri_plugin_shell::init())
+        .plugin(tauri_plugin_dialog::init())
+        .manage(AppState::new())
+        .invoke_handler(tauri::generate_handler![
+            // connections
+            commands::connections::get_connections,
+            commands::connections::save_connection,
+            commands::connections::delete_connection,
+            commands::connections::test_connection,
+            commands::connections::connect,
+            commands::connections::switch_database,
+            commands::connections::disconnect,
+            // queries
+            commands::queries::execute_query,
+            // schema
+            commands::schema::list_databases,
+            commands::schema::list_schemas,
+            commands::schema::list_tables,
+            commands::schema::list_views,
+            commands::schema::list_functions,
+            commands::schema::list_indexes,
+            commands::schema::list_sequences,
+            commands::schema::get_table_columns,
+            commands::schema::get_table_constraints,
+            commands::schema::get_table_indexes,
+            // data
+            commands::data::get_table_data,
+            commands::data::update_row,
+            commands::data::insert_row,
+            commands::data::delete_rows,
+            // export
+            commands::export::export_csv,
+            commands::export::export_json,
+        ])
+        .run(tauri::generate_context!())
+        .expect("error while running tauri application");
+}