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/src/api.ts

@@ -5,6 +5,7 @@ import type {
   Column,
   Metrics,
   MetricsFilter,
+  Sticker,
   User,
 } from "./types";
 
@@ -93,6 +94,13 @@ export function deleteCard(id: string): Promise<void> {
   return fetchJSON(`/cards/${id}`, { method: "DELETE" });
 }
 
+export function updateCardStickers(id: string, stickers: Sticker[]): Promise<void> {
+  return fetchJSON(`/cards/${id}/stickers`, {
+    method: "PUT",
+    body: JSON.stringify({ stickers }),
+  });
+}
+
 export function listTrash(): Promise<Card[]> {
   return fetchJSON("/trash");
 }