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/layout/TabBar.tsx

@@ -1,7 +1,7 @@
 import { useAppStore } from "@/stores/app-store";
 import { useConnections } from "@/hooks/use-connections";
 import { ScrollArea } from "@/components/ui/scroll-area";
-import { X, Table2, Code, Columns, Users, Activity, Search } from "lucide-react";
+import { X, Table2, Code, Columns, Users, Activity, Search, GitFork } from "lucide-react";
 
 export function TabBar() {
   const { tabs, activeTabId, setActiveTabId, closeTab } = useAppStore();
@@ -16,6 +16,7 @@ export function TabBar() {
     roles: <Users className="h-3 w-3" />,
     sessions: <Activity className="h-3 w-3" />,
     lookup: <Search className="h-3 w-3" />,
+    erd: <GitFork className="h-3 w-3" />,
   };
 
   return (