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/tauri.conf.json

@@ -0,0 +1,44 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "productName": "Tusk",
+  "version": "0.1.0",
+  "identifier": "com.tusk.app",
+  "build": {
+    "frontendDist": "../dist",
+    "devUrl": "http://localhost:5173",
+    "beforeDevCommand": "npm run dev",
+    "beforeBuildCommand": "npm run build"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "Tusk",
+        "width": 1280,
+        "height": 800,
+        "resizable": true,
+        "fullscreen": false,
+        "minWidth": 800,
+        "minHeight": 600
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  },
+  "plugins": {
+    "shell": {
+      "open": true
+    }
+  }
+}