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/commands/export.rs

@@ -0,0 +1,58 @@
+use crate::error::{TuskError, TuskResult};
+use serde_json::Value;
+use std::fs::File;
+use std::io::Write;
+
+#[tauri::command]
+pub async fn export_csv(
+    path: String,
+    columns: Vec<String>,
+    rows: Vec<Vec<Value>>,
+) -> TuskResult<()> {
+    let file = File::create(&path)?;
+    let mut wtr = csv::Writer::from_writer(file);
+
+    wtr.write_record(&columns)
+        .map_err(|e| TuskError::Custom(e.to_string()))?;
+
+    for row in &rows {
+        let record: Vec<String> = row
+            .iter()
+            .map(|v| match v {
+                Value::Null => String::new(),
+                Value::String(s) => s.clone(),
+                Value::Bool(b) => b.to_string(),
+                Value::Number(n) => n.to_string(),
+                _ => v.to_string(),
+            })
+            .collect();
+        wtr.write_record(&record)
+            .map_err(|e| TuskError::Custom(e.to_string()))?;
+    }
+
+    wtr.flush().map_err(|e| TuskError::Custom(e.to_string()))?;
+    Ok(())
+}
+
+#[tauri::command]
+pub async fn export_json(
+    path: String,
+    columns: Vec<String>,
+    rows: Vec<Vec<Value>>,
+) -> TuskResult<()> {
+    let objects: Vec<serde_json::Map<String, Value>> = rows
+        .iter()
+        .map(|row| {
+            columns
+                .iter()
+                .zip(row.iter())
+                .map(|(col, val)| (col.clone(), val.clone()))
+                .collect()
+        })
+        .collect();
+
+    let json = serde_json::to_string_pretty(&objects)?;
+    let mut file = File::create(&path)?;
+    file.write_all(json.as_bytes())?;
+    Ok(())
+}