refactor(bootstrap): factorize UI server start/stop logic

refactor(bootstrap): factorize UI server start/stop logic

Extract idempotent helpers and remove duplication across the four
UI server lifecycle call sites:

- public startUIServer() now contains the actual logic instead of
  delegating to a private wrapper; start() calls it directly.
- new private stopUIServer() helper replaces the two inline stop
  blocks in gracefulShutdown() and restart().
- restart() guards syncUIServerTemplates() on uiServerStarted to
  avoid redundant template sync (twice when re-enabling, wasted
  call when the UI server is disabled or stopped). startUIServer()
  performs the sync itself when it actually starts the server.

No public API change. Behavior preserved.