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/components/workspace/TabContent.tsx

@@ -5,6 +5,7 @@ import { TableStructure } from "@/components/table-viewer/TableStructure";
 import { RoleManagerView } from "@/components/management/RoleManagerView";
 import { SessionsView } from "@/components/management/SessionsView";
 import { EntityLookupPanel } from "@/components/lookup/EntityLookupPanel";
+import { ErdDiagram } from "@/components/erd/ErdDiagram";
 
 export function TabContent() {
   const { tabs, activeTabId, updateTab } = useAppStore();
@@ -72,6 +73,14 @@ export function TabContent() {
               />
             );
             break;
+          case "erd":
+            content = (
+              <ErdDiagram
+                connectionId={tab.connectionId}
+                schema={tab.schema!}
+              />
+            );
+            break;
           default:
             content = null;
         }