fix: graceful login error handling and remove hard redirects

- LoginPage.tsx: proper error handling for network errors, rate limiting
  (429), MFA required, account disabled, and server errors
- LoginPage.tsx: dismissible error alerts with onClose
- LoginPage.tsx: added 🐉 branding to login title
- client.ts: removed window.location.href hard redirects on auth failure
  (now uses React state-based logout instead of full page reload)
- client.ts: auth errors now propagate naturally through React Router

frontend/src/api/client.ts

@@ -60,10 +60,8 @@ apiClient.interceptors.response.use(
     isRefreshing = true
 
     const { refreshToken, setTokens, logout } = useAuthStore.getState()
-
     if (!refreshToken) {
       logout()
-      window.location.href = '/login'
       return Promise.reject(error)
     }
 
@@ -78,7 +76,6 @@ apiClient.interceptors.response.use(
     } catch (refreshError) {
       processQueue(refreshError, null)
       logout()
-      window.location.href = '/login'
       return Promise.reject(refreshError)
     } finally {
       isRefreshing = false