* mini: resolve admin credentials from security.toml and env vars
weed mini started the admin UI without resolving admin.user/admin.password
(and the read-only pair) from security.toml [admin] or WEED_ADMIN_* env vars,
so the only way to protect the UI was the -admin.password flag. The standalone
weed admin command applies these fallbacks in runAdmin via applyViperFallback;
the mini path calls startAdminServer directly and skipped it, leaving
authRequired false and the UI unauthenticated.
* mini: load admin.toml maintenance settings
The mini admin path runs ApplyMaintenanceConfigFromToml (via startAdminServer)
against the global viper, but runMini never merged admin.toml, so file-based
maintenance task settings ([maintenance.vacuum], .balance, .erasure_coding)
were ignored under mini while the standalone weed admin honored them. Load it
alongside master/volume config.
* mini: support -admin.urlPrefix for the admin UI
Expose the reverse-proxy subdirectory prefix that the standalone weed admin
already supports, so the mini admin UI can run under e.g. /seaweedfs. The
prefix is normalized the same way and passed through to startAdminServer.
