* feat: add flags to disable WebDAV and Admin UI in weed mini
- Add -webdav flag (default: true) to optionally disable WebDAV server
- Add -admin.ui flag (default: true) to optionally disable Admin UI only (server still runs)
- Conditionally skip WebDAV service startup based on flag
- Pass disableUI flag to SetupRoutes to skip UI route registration
- Admin server still runs for gRPC and API access when UI is disabled
Addresses issue from https://github.com/seaweedfs/seaweedfs/pull/7833#issuecomment-3711924150
* refactor: use positive enableUI parameter instead of disableUI across admin server and handlers
* docs: update mini welcome message to list enabled components
* chore: remove unused welcomeMessageTemplate constant
* docs: split S3 credential message into separate sb.WriteString calls
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