Add dynamic auth configuration endpoint

[kargig]

Implement GET /api/v1/auth/config to inform the frontend of enabled auth providers (Google, Facebook) and their public client/app IDs based on server-side environment variables.

Update Login, Register, and Profile pages to dynamically show or hide social login buttons based on backend capability, preventing the Facebook button from appearing when server credentials are not set.

[github-actions]

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🧪 PR contains tests

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⚡ Recommended focus areas for review Possible Issue The Facebook account-linking block is now gated on isFacebookEnabled, which is derived from authConfig?.facebook?.enabled. When the backend config query has not yet resolved (or fails), authConfig is undefined, so isFacebookEnabled is false. A user who already has user.facebook_id set will no longer see the "Facebook Connected" / "Disconnect" UI until the config request succeeds. If the config endpoint is unreachable, the user cannot disconnect their linked Facebook account at all. Consider showing the connected state whenever user?.facebook_id is truthy, independent of provider enablement. {!user?.google_id && (user?.facebook_id || isFacebookEnabled) && ( Stale Closure handleFacebookLogin now calls facebookAuth.signIn(fbAppId), but fbAppId is computed from authConfig which resolves asynchronously. The useCallback dependency array includes fbAppId, so the callback is recreated when config loads; however, if the user clicks the Facebook button before the config query resolves, fbAppId falls back to import.meta.env.VITE_FACEBOOK_APP_ID. If that env var is unset, signIn is called with undefined and throws 'Facebook App ID not configured.' The button is only rendered when isFacebookEnabled is true, which requires fbAppId to be truthy, so this is mostly guarded — but the same pattern in Register.jsx and Profile.jsx should be verified for consistency. const token = await facebookAuth.signIn(fbAppId);