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/shared/types/effects.ts

@@ -3,6 +3,7 @@ export type EffectToolType =
   | 'fire'
   | 'rain'
   | 'water'
+  | 'darkness'
   | 'lightning'
   | 'sunbeam'
   | 'poisonCloud'
@@ -14,12 +15,14 @@ export type EffectInstanceType =
   | 'fire'
   | 'rain'
   | 'water'
+  | 'darkness'
   | 'lightning'
   | 'sunbeam'
   | 'poisonCloud'
   | 'freeze'
   | 'scorch'
-  | 'ice';
+  | 'ice'
+  | 'shadow';
 
 /** Нормализованные координаты (0..1) относительно области предпросмотра/презентации. */
 export type NPoint = { x: number; y: number; tMs: number; pressure?: number };
@@ -65,6 +68,13 @@ export type WaterInstance = EffectInstanceBase & {
   lifetimeMs: number | null;
 };
 
+export type DarknessInstance = EffectInstanceBase & {
+  type: 'darkness';
+  at: { x: number; y: number };
+  intensity: number;
+  lifetimeMs: number;
+};
+
 export type LightningInstance = EffectInstanceBase & {
   type: 'lightning';
   start: { x: number; y: number };
@@ -119,17 +129,28 @@ export type IceInstance = EffectInstanceBase & {
   lifetimeMs: number | null;
 };
 
+export type ShadowInstance = EffectInstanceBase & {
+  /** Внутренний инстанс: тёмное пятно после эффекта «Тьма». */
+  type: 'shadow';
+  at: { x: number; y: number };
+  radiusN: number;
+  opacity: number;
+  lifetimeMs: number | null;
+};
+
 export type EffectInstance =
   | FogInstance
   | FireInstance
   | RainInstance
   | WaterInstance
+  | DarknessInstance
   | LightningInstance
   | SunbeamInstance
   | PoisonCloudInstance
   | FreezeInstance
   | ScorchInstance
-  | IceInstance;
+  | IceInstance
+  | ShadowInstance;
 
 export type EffectToolState = {
   tool: EffectToolType;