Implement OAuth login functionality and enhance documentation

- Added support for Google OAuth login, including new API endpoints for OAuth providers and callbacks.
- Updated user authentication logic to handle OAuth-only users.
- Enhanced README.md and deployment documentation with OAuth setup instructions.
- Modified frontend components to include OAuth login options and improved error handling.
- Updated configuration files to include new environment variables for OAuth integration.

frontend-nuxt/pages/login.vue

@@ -4,6 +4,18 @@
       <div class="card-body">
         <h1 class="card-title justify-center text-2xl">HnH Map</h1>
         <p class="text-center text-base-content/70 text-sm">Log in to continue</p>
+        <div v-if="(oauthProviders ?? []).length" class="flex flex-col gap-2">
+          <a
+            v-for="p in (oauthProviders ?? [])"
+            :key="p"
+            :href="api.oauthLoginUrl(p, redirect || undefined)"
+            class="btn btn-outline gap-2"
+          >
+            <span v-if="p === 'google'">Login with Google</span>
+            <span v-else>Login with {{ p }}</span>
+          </a>
+          <div class="divider text-sm">or</div>
+        </div>
         <form @submit.prevent="submit" class="flex flex-col gap-4">
           <div class="form-control">
             <label class="label" for="user">User</label>
@@ -40,15 +52,23 @@ const user = ref('')
 const pass = ref('')
 const error = ref('')
 const loading = ref(false)
+const oauthProviders = ref<string[]>([])
 const router = useRouter()
+const route = useRoute()
 const api = useMapApi()
 
+const redirect = computed(() => (route.query.redirect as string) || '')
+
+onMounted(async () => {
+  oauthProviders.value = await api.oauthProviders()
+})
+
 async function submit() {
   error.value = ''
   loading.value = true
   try {
     await api.login(user.value, pass.value)
-    await router.push('/profile')
+    await router.push(redirect.value || '/profile')
   } catch (e: unknown) {
     error.value = e instanceof Error ? e.message : 'Login failed'
   } finally {