feat(mobile): OAuth2 mobile endpoint and Mail Settings field (#485)

[krantheman]

Summary

• Adds mail/api/mobile.py with two endpoints:
  • get_client_id (allow_guest=True) — called by the mobile app before login to validate the site and fetch the OAuth client ID, app name, logo, and sitename. Returns only non-sensitive public config; reviewed and marked # nosemgrep for the guest-whitelisted-method rule (same pattern as Raven's equivalent endpoint).
  • create_oauth_client (requires Mail Settings write permission) — auto-creates or updates a Frappe OAuth Client doc and stores its ID in Mail Settings, so admins don't need to configure it manually
• Adds mobile_oauth_client (Link → OAuth Client) field to Mail Settings under a new Mobile tab
• Adds a Setup Mobile OAuth Client button (Mail Settings → Actions) that calls create_oauth_client and reloads the form
• Redirect URI is com.frappe.mail://oauth — matches the iOS Info.plist and Android AndroidManifest.xml stubs added in Mobile: NativeScript + Vue 3 project scaffolding #484

Modeled on Raven's mobile-login approach (guest get_client_id discovery + admin create_oauth_client, Authorization Code flow against Frappe's OAuth provider). Note: mail users are identified by is_jmap_configured (a User Settings check), not a role, so the OAuth client is not role-gated — the mail APIs already enforce is_jmap_configured.

Admin setup flow

1. Open Mail Settings → Mobile tab in Frappe Desk
2. Click Setup Mobile OAuth Client under Actions — creates/updates the OAuth Client automatically and stores its ID
3. Mobile app users can now enter the site URL and the app will fetch the client_id from get_client_id

Test plan

Verified on a dev site (bench migrate + console/HTTP):

• get_client_id raises DoesNotExistError when no OAuth client is configured
• After create_oauth_client, get_client_id returns client_id, app_name, logo, sitename (also confirmed over HTTP as guest — no auth)
• create_oauth_client raises PermissionError for users without Mail Settings write permission
• Mail Settings shows the Mobile tab + OAuth Client link field after bench migrate
• The created OAuth Client has the expected config (redirect com.frappe.mail://oauth, Authorization Code / Code, scopes = all openid)

Closes #485

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