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/error.rs

@@ -0,0 +1,33 @@
+use serde::Serialize;
+
+#[derive(Debug, thiserror::Error)]
+pub enum TuskError {
+    #[error("Database error: {0}")]
+    Database(#[from] sqlx::Error),
+
+    #[error("IO error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("Serialization error: {0}")]
+    Serde(#[from] serde_json::Error),
+
+    #[error("Connection not found: {0}")]
+    ConnectionNotFound(String),
+
+    #[error("Not connected: {0}")]
+    NotConnected(String),
+
+    #[error("{0}")]
+    Custom(String),
+}
+
+impl Serialize for TuskError {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(&self.to_string())
+    }
+}
+
+pub type TuskResult<T> = Result<T, TuskError>;