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/lib/tauri.ts

@@ -31,6 +31,13 @@ export const disconnectDb = (id: string) =>
 export const switchDatabase = (config: ConnectionConfig, database: string) =>
   invoke<void>("switch_database", { config, database });
 
+// Read-Only
+export const setReadOnly = (connectionId: string, readOnly: boolean) =>
+  invoke<void>("set_read_only", { connectionId, readOnly });
+
+export const getReadOnly = (connectionId: string) =>
+  invoke<boolean>("get_read_only", { connectionId });
+
 // Queries
 export const executeQuery = (connectionId: string, sql: string) =>
   invoke<QueryResult>("execute_query", { connectionId, sql });