feat(web): the Display tab is an ES-module page, with a page-visibility service (stage 4) (#771)

* feat(web): the Display tab is an ES-module page, with a page-visibility service (stage 4)

- core/visibility.js: each page gets ctx.visibility (whileVisible, every,
  isVisible). Work registered there runs only while the page's tab is
  active and the browser tab visible, and ends when the page is swapped
  out. It reads the active tab from window.LEDVisibility, so it agrees
  with the classic partials. The registry gained a mountContext option for
  per-mount services.
- pages/display.js replaces display.html's two inline scripts. The 5 s
  sync status poll runs through ctx.visibility.every; the status and
  scroll-speed hint requests go through ctx.api with ctx.signal, as does
  the Vegas order widget's plugin-list request. The Advanced toggle is a
  delegated data-action; window.updateSyncUI is a deprecated alias.
- New DOM suites test_visibility_service.js and test_display_page.js;
  test_display_partial_ids.js imports the module.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* refactor(web): no computed keys in the Display page's readout and destroy

Codacy's object-injection rule flagged v[id] and ctx.state[name].

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/js/run_all.js

@@ -37,7 +37,8 @@ const DOM = ['dom/test_installed_dom.js', 'dom/test_store_dom.js', 'dom/test_no_
              'dom/test_tools_sections.js', 'dom/test_cache_page.js',
              'dom/test_durations_page.js', 'dom/test_operation_history_page.js',
              'dom/test_raw_json_page.js', 'dom/test_backup_restore_page.js',
-             'dom/test_schedule_page.js', 'dom/test_general_page.js'];
+             'dom/test_schedule_page.js', 'dom/test_general_page.js',
+             'dom/test_visibility_service.js', 'dom/test_display_page.js'];
 
 function reachable(url) {
   return new Promise(res => {