fix(web): address self-review findings on #105 (settings cache + brittle reveal)

- Fix the gesture enable-reload race: PUT /api/settings writes settings.json
  without invalidating WebServer's 2s _settingsCache, and the toggle reloads
  ~400ms after save — within the TTL — so renderIndexHtml could render the
  pre-toggle state (bundle not injected until a 2nd reload). renderIndexHtml
  now reads settings via readSettings(true), a fresh read that bypasses the
  cache; readSettings gains a forceFresh param.
- Replace the brittle multi-monitor reveal (string match on the button's
  aria-label + inline style) with a stable `btn-multimonitor--hidden` class
  marker: the template carries the class, the server strips it when the setting
  is on, and applyHeaderVisibilitySettings()/solo-mode CSS toggle the same class.
  Editing the button's copy no longer silently breaks the reveal.
- Test: test/render-index-html.test.ts (reveal, solo injection + escaping,
  gesture availability vs. enablement, fresh-read wiring).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/styles.css

@@ -846,6 +846,13 @@ body {
   transform: rotate(45deg);
 }
 
+/* Multi-monitor header button: hidden by default (opt-in via App Settings →
+   Display → "Header Displays"). The server strips this class at render when the
+   setting is on; the client toggles it live on save. */
+.btn-multimonitor--hidden {
+  display: none !important;
+}
+
 .btn-icon-header.btn-settings {
   width: 30px;
   height: 30px;