feat: add ER diagram and enhance TableStructure with FK details, triggers, comments

- Add interactive ER diagram with ReactFlow + dagre auto-layout, accessible
  via right-click context menu on schema nodes in the sidebar
- Enhance TableStructure: column comments, FK referenced table/columns,
  ON UPDATE/DELETE rules, new Triggers tab
- Backend: rewrite get_table_constraints using pg_constraint for proper
  composite FK support, add get_table_triggers and get_schema_erd commands

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -59,6 +59,8 @@ pub fn run() {
             commands::schema::get_table_indexes,
             commands::schema::get_completion_schema,
             commands::schema::get_column_details,
+            commands::schema::get_table_triggers,
+            commands::schema::get_schema_erd,
             // data
             commands::data::get_table_data,
             commands::data::update_row,