feat(kanban): stickers feature + dashboard null guards (#0063)

- backend: Sticker type, idempotent stickers column, PUT /api/cards/:id/stickers, 4 tests
- frontend: emoji-mart picker, toolbar button + ESC, draggable overlay with right-click delete, % coords for resize survival
- dashboard: null guards on metrics arrays

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -12,6 +12,8 @@
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
+    "@emoji-mart/data": "^1.2.1",
+    "@emoji-mart/react": "^1.1.1",
     "@mantine/charts": "^9.1.1",
     "@mantine/core": "^9.0.2",
     "@mantine/dates": "^9.1.1",
@@ -20,6 +22,7 @@
     "@mantine/notifications": "^9.0.2",
     "@tabler/icons-react": "^3.31.0",
     "dayjs": "^1.11.20",
+    "emoji-mart": "^5.6.0",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "react-markdown": "^10.1.0",