feat(08-01): registry foundation for 4 new widget types + Calculator widget (GREEN)

- widget-registry.tsx: extend WidgetType union with calculator/favorites/link/stopwatch
- widget-registry.tsx: add WIDGET_CONSTRAINTS entries with per-widget grid constraints (DASH-11)
- widget-registry.tsx: add SVG icons (CalculatorIcon, FavoritesIcon, LinkIcon, StopwatchIcon)
- widget-registry.tsx: add WIDGET_REGISTRY entries and wire functions for all 4 new types
- calculator-widget.tsx: full arithmetic implementation ported from personal-dashboard
  (parseDisplay, formatNumber, calculate, keyboard handler with stopPropagation)
- widget-catalog-modal.tsx: extend WIDGET_TYPES to include all 8 types
- create-widget.dto.ts: extend @IsIn to accept 8 widget types (T-08-01 mitigated)
- page.tsx: import CalculatorWidget and call wireCalculatorWidget()
- de.json / en.json: add i18n keys for calculator, favorites, link, stopwatch
- All 16 tests passing (GREEN)

apps/api/src/dashboard/dto/create-widget.dto.ts

@@ -7,7 +7,7 @@ import { IsIn, IsObject, IsOptional, IsString } from 'class-validator';
  */
 export class CreateWidgetDto {
   @IsString()
-  @IsIn(['clock', 'search', 'calendar', 'note'])
+  @IsIn(['clock', 'search', 'calendar', 'note', 'calculator', 'favorites', 'link', 'stopwatch'])
   widgetType!: string;
 
   @IsOptional()