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/pnpm-lock.yaml

@@ -17,6 +17,12 @@ importers:
       '@dnd-kit/utilities':
         specifier: ^3.2.2
         version: 3.2.2(react@19.2.5)
+      '@emoji-mart/data':
+        specifier: ^1.2.1
+        version: 1.2.1
+      '@emoji-mart/react':
+        specifier: ^1.1.1
+        version: 1.1.1(emoji-mart@5.6.0)(react@19.2.5)
       '@mantine/charts':
         specifier: ^9.1.1
         version: 9.1.1(@mantine/core@9.1.1(@mantine/hooks@9.1.1(react@19.2.5))(@types/react@19.2.14)(react-dom@19.2.5(react@19.2.5))(react@19.2.5))(@mantine/hooks@9.1.1(react@19.2.5))(react-dom@19.2.5(react@19.2.5))(react@19.2.5)(recharts@2.15.4(react-dom@19.2.5(react@19.2.5))(react@19.2.5))
@@ -41,6 +47,9 @@ importers:
       dayjs:
         specifier: ^1.11.20
         version: 1.11.20
+      emoji-mart:
+        specifier: ^5.6.0
+        version: 5.6.0
       react:
         specifier: ^19.1.0
         version: 19.2.5
@@ -190,6 +199,15 @@ packages:
     peerDependencies:
       react: '>=16.8.0'
 
+  '@emoji-mart/data@1.2.1':
+    resolution: {integrity: sha512-no2pQMWiBy6gpBEiqGeU77/bFejDqUTRY7KX+0+iur13op3bqUsXdnwoZs6Xb1zbv0gAj5VvS1PWoUUckSr5Dw==}
+
+  '@emoji-mart/react@1.1.1':
+    resolution: {integrity: sha512-NMlFNeWgv1//uPsvLxvGQoIerPuVdXwK/EUek8OOkJ6wVOWPUizRBJU0hDqWZCOROVpfBgCemaC3m6jDOXi03g==}
+    peerDependencies:
+      emoji-mart: ^5.2
+      react: ^16.8 || ^17 || ^18
+
   '@esbuild/aix-ppc64@0.25.12':
     resolution: {integrity: sha512-Hhmwd6CInZ3dwpuGTF8fJG6yoWmsToE+vYgD4nytZVxcu1ulHpUQRAB1UJ8+N1Am3Mz4+xOByoQoSZf4D+CpkA==}
     engines: {node: '>=18'}
@@ -781,6 +799,9 @@ packages:
   electron-to-chromium@1.5.351:
     resolution: {integrity: sha512-9D7Iqx8RImSvCnOsj86rCH6eQjZFQoM04Jn6HnZVM0Nu/G58/gmKYQ1d12MZTbjQbQSTGI8nwEy07ErsA2slLA==}
 
+  emoji-mart@5.6.0:
+    resolution: {integrity: sha512-eJp3QRe79pjwa+duv+n7+5YsNhRcMl812EcFVwrnRvYKoNPoQb5qxU8DG6Bgwji0akHdp6D4Ln6tYLG58MFSow==}
+
   esbuild@0.25.12:
     resolution: {integrity: sha512-bbPBYYrtZbkt6Os6FiTLCTFxvq4tt3JKall1vRwshA3fdVztsLAatFaZobhkBC8/BrPetoa0oksYoKXoG4ryJg==}
     engines: {node: '>=18'}
@@ -1491,6 +1512,13 @@ snapshots:
       react: 19.2.5
       tslib: 2.8.1
 
+  '@emoji-mart/data@1.2.1': {}
+
+  '@emoji-mart/react@1.1.1(emoji-mart@5.6.0)(react@19.2.5)':
+    dependencies:
+      emoji-mart: 5.6.0
+      react: 19.2.5
+
   '@esbuild/aix-ppc64@0.25.12':
     optional: true
 
@@ -1943,6 +1971,8 @@ snapshots:
 
   electron-to-chromium@1.5.351: {}
 
+  emoji-mart@5.6.0: {}
+
   esbuild@0.25.12:
     optionalDependencies:
       '@esbuild/aix-ppc64': 0.25.12