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/auth/LoginButton.tsx

@@ -2,10 +2,10 @@
 
 import { Button } from "@mosaic/ui";
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL || "http://localhost:3001";
+const API_URL = process.env.NEXT_PUBLIC_API_URL ?? "http://localhost:3001";
 
-export function LoginButton() {
-  const handleLogin = () => {
+export function LoginButton(): React.JSX.Element {
+  const handleLogin = (): void => {
     // Redirect to the backend OIDC authentication endpoint
     // BetterAuth will handle the OIDC flow and redirect back to the callback
     window.location.assign(`${API_URL}/auth/callback/authentik`);