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>

package.json

@@ -18,10 +18,13 @@
     "@tauri-apps/api": "^2.10.1",
     "@tauri-apps/plugin-dialog": "^2.6.0",
     "@tauri-apps/plugin-shell": "^2.3.5",
+    "@types/dagre": "^0.7.53",
     "@uiw/react-codemirror": "^4.25.4",
+    "@xyflow/react": "^12.10.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
+    "dagre": "^0.8.5",
     "lucide-react": "^0.563.0",
     "next-themes": "^0.4.6",
     "radix-ui": "^1.4.3",