refactor(settings): extract four panels into settings/ modules

static/js/settings.js was 5,721 lines behind a four-name public surface.
static/js/settings/ already existed with dom, registry, search, sidebar,
navigation and lifecycle, so this continues that package rather than
inventing a layout.

Moved verbatim, 407 lines:

  settings/speech.js        initTtsSettings, initSttSettings
  settings/writingStyle.js  initDocumentWritingStyle
  settings/imageModels.js   initImageSettings
  settings/agent.js         initAgentSettings
  settings/api.js           postSettings, lifted from _postSettings

These five were chosen because their only dependencies outside themselves
were el/byId, _postSettings and sortModelIds. Panels with wider reach stay
put: initEmailAccountsSettings, for instance, pulls a 64-declaration closure
covering most of the file, and splitting that is a design change rather than
a move.

settings.js keeps its public surface exactly: open, close,
refreshAiModelEndpoints and the default export, verified in a browser.

Three test updates the move required:

- tests/helpers/test_settings_shell_coordinator.mjs allowlists the real
  modules the coordinator may import, and rejected the new ones. That guard
  working is the reason to trust the rest of this diff.
- two source-introspection tests read settings.js for behaviour that now
  lives in a panel module. They read the whole settings surface now, so the
  next extraction does not break them again.

tests/helpers/test_settings_shell_coordinator.mjs

@@ -20,6 +20,11 @@ const REAL_MODULES = new Set([
   path.join(JS, 'settings/sidebar.js'),
   path.join(JS, 'settings/navigation.js'),
   path.join(JS, 'settings/lifecycle.js'),
+  path.join(JS, 'settings/api.js'),
+  path.join(JS, 'settings/speech.js'),
+  path.join(JS, 'settings/writingStyle.js'),
+  path.join(JS, 'settings/imageModels.js'),
+  path.join(JS, 'settings/agent.js'),
   path.join(JS, 'searchProviderIcons.js'),
 ]);