fix: Resolve all ESLint errors and warnings in web package

Fixes all 542 ESLint problems in the web package to achieve 0 errors and 0 warnings.

Changes:
- Fixed 144 issues: nullish coalescing, return types, unused variables
- Fixed 118 issues: unnecessary conditions, type safety, template literals
- Fixed 79 issues: non-null assertions, unsafe assignments, empty functions
- Fixed 67 issues: explicit return types, promise handling, enum comparisons
- Fixed 45 final warnings: missing return types, optional chains
- Fixed 25 typecheck-related issues: async/await, type assertions, formatting
- Fixed JSX.Element namespace errors across 90+ files

All Quality Rails violations resolved. Lint and typecheck both pass with 0 problems.

Files modified: 118 components, tests, hooks, and utilities

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

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

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