feat: add per-connection read-only mode

Connections default to read-only. SQL editor wraps queries in a
read-only transaction so PostgreSQL rejects mutations. Data mutation
commands (update_row, insert_row, delete_rows) are blocked at the
Rust layer. Toolbar toggle with confirmation dialog lets users
switch to read-write. Badges shown in workspace, table viewer, and
status bar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/commands/queries.rs

@@ -74,16 +74,32 @@ pub async fn execute_query(
     connection_id: String,
     sql: String,
 ) -> TuskResult<QueryResult> {
+    let read_only = state.is_read_only(&connection_id).await;
+
     let pools = state.pools.read().await;
     let pool = pools
         .get(&connection_id)
         .ok_or(TuskError::NotConnected(connection_id))?;
 
     let start = Instant::now();
-    let rows = sqlx::query(&sql)
-        .fetch_all(pool)
-        .await
-        .map_err(TuskError::Database)?;
+    let rows = if read_only {
+        let mut tx = pool.begin().await.map_err(TuskError::Database)?;
+        sqlx::query("SET TRANSACTION READ ONLY")
+            .execute(&mut *tx)
+            .await
+            .map_err(TuskError::Database)?;
+        let result = sqlx::query(&sql)
+            .fetch_all(&mut *tx)
+            .await
+            .map_err(TuskError::Database);
+        tx.rollback().await.map_err(TuskError::Database)?;
+        result?
+    } else {
+        sqlx::query(&sql)
+            .fetch_all(pool)
+            .await
+            .map_err(TuskError::Database)?
+    };
     let execution_time_ms = start.elapsed().as_millis();
 
     let mut columns = Vec::new();