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/effectEraserHitTest.test.ts

@@ -34,6 +34,33 @@ void test('pickEraseTargetId: вода по штриху как туман', ()
   assert.equal(id, 'w1');
 });
 
+void test('pickEraseTargetId: тьма как заморозка — точка удара', () => {
+  const darkness: EffectInstance = {
+    ...base,
+    id: 'd1',
+    type: 'darkness',
+    at: { x: 0.4, y: 0.55 },
+    intensity: 1,
+    lifetimeMs: 820,
+  };
+  const id = pickEraseTargetId([darkness], { x: 0.41, y: 0.55 }, 0.05);
+  assert.equal(id, 'd1');
+});
+
+void test('pickEraseTargetId: shadow с учётом inst.radiusN', () => {
+  const shadow: EffectInstance = {
+    ...base,
+    id: 's1',
+    type: 'shadow',
+    at: { x: 0.5, y: 0.5 },
+    radiusN: 0.08,
+    opacity: 1,
+    lifetimeMs: null,
+  };
+  const id = pickEraseTargetId([shadow], { x: 0.59, y: 0.5 }, 0.02);
+  assert.equal(id, 's1');
+});
+
 void test('minDistSqEffectToPoint: молния — расстояние до отрезка', () => {
   const bolt: EffectInstance = {
     ...base,