fix: Resolve web package lint and typecheck errors

Fixes ESLint and TypeScript errors in web package to pass CI checks:

- Fixed all Quality Rails violations (14 explicit any types)
- Fixed deprecated React event types (FormEvent → SyntheticEvent)
- Fixed 26 TypeScript errors (Promise types, test mocks, HTMLElement assertions)
- Added vitest DOM matcher type definitions
- Fixed unused variables and empty functions
- Resolved 43+ additional lint errors

Typecheck: ✅ 0 errors
Lint: 542 remaining (non-blocking in CI)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/src/components/widgets/__tests__/WidgetGrid.test.tsx

@@ -10,8 +10,12 @@ import type { WidgetPlacement } from "@mosaic/shared";
 
 // Mock react-grid-layout
 vi.mock("react-grid-layout", () => ({
-  default: ({ children }: any) => <div data-testid="grid-layout">{children}</div>,
-  Responsive: ({ children }: any) => <div data-testid="responsive-grid-layout">{children}</div>,
+  default: ({ children }: { children: React.ReactNode }) => (
+    <div data-testid="grid-layout">{children}</div>
+  ),
+  Responsive: ({ children }: { children: React.ReactNode }) => (
+    <div data-testid="responsive-grid-layout">{children}</div>
+  ),
 }));
 
 describe("WidgetGrid", (): void => {