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/renderer/control/ControlApp.tsx

@@ -69,7 +69,7 @@ export function ControlApp() {
   const previewHostRef = useRef<HTMLDivElement | null>(null);
   const previewVideoRef = useRef<HTMLVideoElement | null>(null);
   const brushRef = useRef<{
-    tool: 'fog' | 'fire' | 'rain' | 'water' | 'lightning' | 'sunbeam' | 'poisonCloud' | 'freeze' | 'eraser';
+    tool: 'fog' | 'fire' | 'rain' | 'water' | 'darkness' | 'lightning' | 'sunbeam' | 'poisonCloud' | 'freeze' | 'eraser';
     startN?: { x: number; y: number };
     points?: { x: number; y: number; tMs: number }[];
   } | null>(null);
@@ -133,6 +133,11 @@ export function ControlApp() {
     void getFreezeEffectLifeMs().then(setFreezeDraftLifeMs);
   }, []);
 
+  const [darknessDraftLifeMs, setDarknessDraftLifeMs] = useState(820);
+  useEffect(() => {
+    void getFreezeEffectLifeMs().then(setDarknessDraftLifeMs);
+  }, []);
+
   const [sunbeamDraftLifeMs, setSunbeamDraftLifeMs] = useState(600);
   useEffect(() => {
     void getSunbeamEffectLifeMs().then(setSunbeamDraftLifeMs);
@@ -714,6 +719,37 @@ export function ControlApp() {
         },
       });
     }
+    if (b.tool === 'darkness' && b.points && b.points.length > 0) {
+      const last = b.points[b.points.length - 1];
+      if (last === undefined) return;
+      const at = { x: last.x, y: last.y };
+      const darknessLifeMs = await getFreezeEffectLifeMs();
+      await fx.dispatch({
+        kind: 'instance.add',
+        instance: {
+          id: `dk_${String(createdAtMs)}_${String(seed)}`,
+          type: 'darkness',
+          seed,
+          createdAtMs,
+          at,
+          intensity: Math.max(0.8, Math.min(1.25, tool.intensity * 1.15)),
+          lifetimeMs: darknessLifeMs,
+        },
+      });
+      await fx.dispatch({
+        kind: 'instance.add',
+        instance: {
+          id: `sh_${String(createdAtMs)}_${String(seed)}`,
+          type: 'shadow',
+          seed: seed ^ 0x0d4a0001,
+          createdAtMs,
+          at,
+          radiusN: Math.max(0.03, tool.radiusN * 0.9),
+          opacity: 1,
+          lifetimeMs: null,
+        },
+      });
+    }
     if (b.tool === 'lightning' && b.startN && b.points && b.points.length > 0) {
       const last = b.points[b.points.length - 1];
       if (last === undefined) return;
@@ -879,6 +915,19 @@ export function ControlApp() {
         lifetimeMs: null,
       };
     }
+    if (b.tool === 'darkness' && b.points && b.points.length > 0) {
+      const last = b.points[b.points.length - 1];
+      if (last === undefined) return null;
+      return {
+        id: '__draft__',
+        type: 'darkness' as const,
+        seed,
+        createdAtMs,
+        at: { x: last.x, y: last.y },
+        intensity: Math.max(0.8, Math.min(1.25, tool.intensity * 1.15)),
+        lifetimeMs: darknessDraftLifeMs,
+      };
+    }
     if (b.tool === 'lightning' && b.startN && b.points && b.points.length > 0) {
       const last = b.points[b.points.length - 1];
       if (last === undefined) return null;
@@ -940,6 +989,7 @@ export function ControlApp() {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [
     draftFxTick,
+    darknessDraftLifeMs,
     freezeDraftLifeMs,
     poisonDraftLifeMs,
     sunbeamDraftLifeMs,
@@ -1073,6 +1123,15 @@ export function ControlApp() {
                   >
                     <span className={styles.iconGlyph}>❄️</span>
                   </Button>
+                  <Button
+                    variant={tool.tool === 'darkness' ? 'primary' : 'ghost'}
+                    iconOnly
+                    title={t('control.darkness')}
+                    ariaLabel={t('control.darkness')}
+                    onClick={() => void fx.dispatch({ kind: 'tool.set', tool: { ...tool, tool: 'darkness' } })}
+                  >
+                    <span className={styles.iconGlyph}>🌑</span>
+                  </Button>
                   <Button
                     variant={tool.tool === 'poisonCloud' ? 'primary' : 'ghost'}
                     iconOnly