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/capabilities/default.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "enables the default permissions",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "shell:allow-open",
+    "dialog:allow-save",
+    "dialog:allow-open"
+  ]
+}