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/composables/useMapApi.ts

@@ -53,11 +53,33 @@ export function useMapApi() {
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ user, pass }),
     })
-    if (res.status === 401) throw new Error('Unauthorized')
+    if (res.status === 401) {
+      const data = (await res.json().catch(() => ({}))) as { error?: string }
+      throw new Error(data.error || 'Unauthorized')
+    }
     if (!res.ok) throw new Error(`API ${res.status}`)
     return res.json() as Promise<MeResponse>
   }
 
+  /** OAuth login URL for redirect (full page navigation). */
+  function oauthLoginUrl(provider: string, redirect?: string): string {
+    const url = new URL(`${apiBase}/oauth/${provider}/login`)
+    if (redirect) url.searchParams.set('redirect', redirect)
+    return url.toString()
+  }
+
+  /** List of configured OAuth providers. */
+  async function oauthProviders(): Promise<string[]> {
+    try {
+      const res = await fetch(`${apiBase}/oauth/providers`, { credentials: 'include' })
+      if (!res.ok) return []
+      const data = await res.json()
+      return Array.isArray(data) ? data : []
+    } catch {
+      return []
+    }
+  }
+
   async function logout() {
     await fetch(`${apiBase}/logout`, { method: 'POST', credentials: 'include' })
   }
@@ -183,6 +205,8 @@ export function useMapApi() {
     login,
     logout,
     me,
+    oauthLoginUrl,
+    oauthProviders,
     setupRequired,
     meTokens,
     mePassword,