fix: resolve CI build and test failures from noImplicitReturns and mockReset

Add explicit `return undefined` to useEffect callbacks that conditionally
return cleanup functions (pde, shortcuts-modal, command-palette,
webgpu-heatmap). Remove `mockReset: true` from API vitest config since it
clears module-level mock implementations between tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ABCrimson

apps/api/vitest.config.ts

@@ -7,7 +7,6 @@ export default defineConfig({
     passWithNoTests: true,
     restoreMocks: true,
     clearMocks: true,
-    mockReset: true,
     include: ['src/**/*.{test,spec}.{ts,tsx}'],
     exclude: ['node_modules', 'dist'],
     coverage: {

apps/web/app/[locale]/pde/page.tsx

@@ -517,6 +517,7 @@ export default function PDESolverPage() {
         }
       };
     }
+    return undefined;
   }, [isPlaying, simulationData, playbackSpeed]);
 
   /**

apps/web/components/calculator/shortcuts-modal.tsx

@@ -313,6 +313,7 @@ export function ShortcutsModal({ registerGlobalHotkey = true }: ShortcutsModalPr
         }
       };
     }
+    return undefined;
   }, []);
 
   const dismissOnboarding = useCallback(() => {

apps/web/components/layout/command-palette.tsx

@@ -600,6 +600,7 @@ export function CommandPalette({ className }: CommandPaletteProps) {
       });
       return () => cancelAnimationFrame(raf);
     }
+    return undefined;
   }, [open]);
 
   // Reset activeIndex when results change

apps/web/components/plots/webgpu-heatmap.tsx

@@ -890,6 +890,7 @@ export function WebGPUHeatmap({
         { width: cols, height: rows },
       );
     }
+    return undefined;
   }, [data, gpuSolverEnabled, dataVersion]);
 
   // -------------------------------------------------------------------------
@@ -1048,6 +1049,7 @@ export function WebGPUHeatmap({
       size: rows * cols * 4,
       usage: GPUBufferUsage.MAP_READ | GPUBufferUsage.COPY_DST,
     });
+    return undefined;
   }, [gpuSolverEnabled, equationType, gpuAvailable, dataVersion, data, solverAlpha]);
 
   // -------------------------------------------------------------------------