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/components/KanbanColumn.tsx

@@ -54,6 +54,11 @@ interface Props {
   onShowHistory: (card: Card) => void;
   onToggleCardLock: (id: string, locked: boolean) => void;
   onAssignCard: (id: string, assignee_id: string | null) => void;
+  activeSticker?: string | null;
+  onAddSticker?: (cardId: string, x: number, y: number) => void;
+  onRemoveSticker?: (cardId: string, index: number) => void;
+  onMoveSticker?: (cardId: string, index: number, x: number, y: number) => void;
+  onCommitSticker?: (cardId: string) => void;
   users: User[];
   usersById: Map<string, User>;
 }
@@ -76,6 +81,11 @@ function KanbanColumnImpl({
   onShowHistory,
   onToggleCardLock,
   onAssignCard,
+  activeSticker,
+  onAddSticker,
+  onRemoveSticker,
+  onMoveSticker,
+  onCommitSticker,
   users,
   usersById,
 }: Props) {
@@ -408,6 +418,11 @@ function KanbanColumnImpl({
                     users={users}
                     assignee={c.assignee_id ? usersById.get(c.assignee_id) : undefined}
                     inDoneColumn={column.is_done}
+                    activeSticker={activeSticker}
+                    onAddSticker={onAddSticker}
+                    onRemoveSticker={onRemoveSticker}
+                    onMoveSticker={onMoveSticker}
+                    onCommitSticker={onCommitSticker}
                   />
                 ))}
               </Stack>