fix(#411): remediate frontend review findings — wire fetchWithRetry, fix error handling

- Wire fetchWithRetry into login page config fetch (was dead code)
- Remove duplicate ERROR_CODE_MESSAGES, use parseAuthError from auth-errors.ts
- Fix OAuth sign-in fire-and-forget: add .catch() with PDA error + loading reset
- Fix credential login catch: use parseAuthError for better error messages
- Add user feedback when auth config fetch fails (was silent degradation)
- Fix sign-out failure: use logAuthError and set authError state
- Enable fetchWithRetry production logging for retry visibility

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/lib/auth/auth-context.tsx

@@ -129,7 +129,8 @@ export function AuthProvider({ children }: { children: ReactNode }): React.JSX.E
     try {
       await apiPost("/auth/sign-out");
     } catch (error) {
-      console.error("Sign out error:", error);
+      logAuthError("Sign out request did not complete", error);
+      setAuthError("network");
     } finally {
       setUser(null);
       expiresAtRef.current = null;