feat: implement Story 1.4 — recent view and drag-and-drop organization

Add sortOrder column to diagrams, extend PATCH endpoint with projectId
and sortOrder fields, add POST /diagrams/reorder bulk endpoint with
ownership verification and duplicate ID validation. Enhance RecentList
with lastAiMessage preview subtitle. Implement @dnd-kit drag-and-drop
in ProjectTree sidebar with cross-project moves, intra-project reorder,
optimistic updates, drag overlay, drop indicators, and keyboard/pointer
sensor support. Context-aware GET ordering: sortOrder for project views,
updatedAt for recent/all views. 141 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/package.json

@@ -16,6 +16,9 @@
   "dependencies": {
     "@ai-sdk/react": "2.0.86",
     "@anaralabs/lector": "3.7.3",
+    "@dnd-kit/core": "6.3.1",
+    "@dnd-kit/sortable": "10.0.0",
+    "@dnd-kit/utilities": "3.2.2",
     "@formatjs/intl-localematcher": "0.6.2",
     "@hookform/resolvers": "5.2.2",
     "@next/bundle-analyzer": "16.0.10",