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/lib.rs

@@ -19,6 +19,8 @@ pub fn run() {
             commands::connections::connect,
             commands::connections::switch_database,
             commands::connections::disconnect,
+            commands::connections::set_read_only,
+            commands::connections::get_read_only,
             // queries
             commands::queries::execute_query,
             // schema