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/providers/ThemeProvider.tsx

@@ -34,7 +34,10 @@ interface ThemeProviderProps {
   defaultTheme?: Theme;
 }
 
-export function ThemeProvider({ children, defaultTheme = "system" }: ThemeProviderProps) {
+export function ThemeProvider({
+  children,
+  defaultTheme = "system",
+}: ThemeProviderProps): React.JSX.Element {
   const [theme, setThemeState] = useState<Theme>(defaultTheme);
   const [resolvedTheme, setResolvedTheme] = useState<"light" | "dark">("dark");
   const [mounted, setMounted] = useState(false);
@@ -64,7 +67,7 @@ export function ThemeProvider({ children, defaultTheme = "system" }: ThemeProvid
     if (!mounted || theme !== "system") return;
 
     const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
-    const handleChange = (e: MediaQueryListEvent) => {
+    const handleChange = (e: MediaQueryListEvent): void => {
       setResolvedTheme(e.matches ? "dark" : "light");
       document.documentElement.classList.remove("light", "dark");
       document.documentElement.classList.add(e.matches ? "dark" : "light");
@@ -112,7 +115,7 @@ export function ThemeProvider({ children, defaultTheme = "system" }: ThemeProvid
   );
 }
 
-export function useTheme() {
+export function useTheme(): ThemeContextValue {
   const context = useContext(ThemeContext);
   if (!context) {
     throw new Error("useTheme must be used within a ThemeProvider");