Add Outlook unread-mail widget with device-code login

- outlook-auth.js: one-time device-code flow against Microsoft login;
  stores the refresh token in the outlook-tokens compose volume
- update-outlook.js: per-minute cron fetching the unread count and the
  latest unread messages via Microsoft Graph; caches access tokens and
  persists rotated refresh tokens
- Widget is hidden until the mailbox is actually authorized, so it
  stays out of sight while (admin) consent is pending
- New external card for Outlook (Office 365) with the official icon
- compose: pass OUTLOOK_CLIENT_ID/OUTLOOK_TENANT/GITEA_TOKEN from .env
- README: Entra ID app registration steps and the tenant-ID hint for
  directories not resolvable via the organizations endpoint

.gitignore

@@ -1,5 +1,6 @@
 node_modules
 api/node_modules
+api/tokens
 *.tgz
 *.log
 .env