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/models/query_result.rs

@@ -0,0 +1,23 @@
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct QueryResult {
+    pub columns: Vec<String>,
+    pub types: Vec<String>,
+    pub rows: Vec<Vec<Value>>,
+    pub row_count: usize,
+    pub execution_time_ms: u128,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PaginatedQueryResult {
+    pub columns: Vec<String>,
+    pub types: Vec<String>,
+    pub rows: Vec<Vec<Value>>,
+    pub row_count: usize,
+    pub execution_time_ms: u128,
+    pub total_rows: i64,
+    pub page: u32,
+    pub page_size: u32,
+}