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`);

apps/web/src/components/auth/LogoutButton.test.tsx

@@ -6,7 +6,7 @@ import { LogoutButton } from "./LogoutButton";
 // Mock next/navigation
 const mockPush = vi.fn();
 vi.mock("next/navigation", () => ({
-  useRouter: () => ({
+  useRouter: (): { push: typeof mockPush } => ({
     push: mockPush,
   }),
 }));
@@ -14,7 +14,7 @@ vi.mock("next/navigation", () => ({
 // Mock auth context
 const mockSignOut = vi.fn();
 vi.mock("@/lib/auth/auth-context", () => ({
-  useAuth: () => ({
+  useAuth: (): { signOut: typeof mockSignOut } => ({
     signOut: mockSignOut,
   }),
 }));
@@ -51,7 +51,9 @@ describe("LogoutButton", (): void => {
     mockSignOut.mockRejectedValue(new Error("Sign out failed"));
 
     // Suppress console.error for this test
-    const consoleErrorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
+    const consoleErrorSpy = vi.spyOn(console, "error").mockImplementation(() => {
+      // Intentionally empty - suppressing error output
+    });
 
     render(<LogoutButton />);
 

apps/web/src/components/auth/LogoutButton.tsx

@@ -9,11 +9,14 @@ interface LogoutButtonProps {
   className?: string;
 }
 
-export function LogoutButton({ variant = "secondary", className }: LogoutButtonProps) {
+export function LogoutButton({
+  variant = "secondary",
+  className,
+}: LogoutButtonProps): React.JSX.Element {
   const router = useRouter();
   const { signOut } = useAuth();
 
-  const handleSignOut = async () => {
+  const handleSignOut = async (): Promise<void> => {
     try {
       await signOut();
     } catch (error) {