feat(effects): add Darkness action effect like freeze

Adds a fullscreen darkness vignette on click and a permanent shadow spot with a black radial gradient (40% edge, 100% center). Includes control panel UI, i18n, eraser support, and tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/main/effects/effectsStore.ts

@@ -46,8 +46,8 @@ export class EffectsStore {
     const before = this.state.instances.length;
     const kept = this.state.instances.filter((i) => {
       // Пятно льда не истекает по таймеру (только «очистить все» или ластик в UI).
-      if (i.type === 'ice') return true;
-      if (i.type === 'lightning' || i.type === 'sunbeam' || i.type === 'poisonCloud') {
+      if (i.type === 'ice' || i.type === 'shadow') return true;
+      if (i.type === 'lightning' || i.type === 'sunbeam' || i.type === 'poisonCloud' || i.type === 'darkness') {
         return now - i.createdAtMs < i.lifetimeMs;
       }
       if (i.type === 'scorch') {