feat(attachments): make the header attachments button opt-in (default OFF)

The COD-39 attachments button was hard-visible in the header — first on
mobile, then (after the mobile-only hide) still on desktop. Make it a
proper opt-in App Settings → Display toggle ("Attachments Button"),
default OFF everywhere, mirroring the Response Viewer button:

- index.html: button ships with the `btn-attachments-history--hidden`
  marker; new settings checkbox #appSettingsShowAttachmentsButton.
- styles.css: base `display:inline-flex !important` + a more-specific
  `--hidden` rule (same pattern as the response viewer).
- settings-ui.js: load/save/getDefaultSettings(false) + a live toggle in
  applyHeaderVisibilitySettings. Per-device and NON-leaking — added to
  displayKeys AND stripped from the server payload, so enabling it on
  desktop never makes it appear on mobile (or any other device). No
  server-side render step (purely client display, like the eye button).
- mobile.css: dropped the now-redundant phone-only hide — the opt-in
  marker hides it everywhere by default; the per-device toggle governs
  both desktop and phone.

Tests updated: the CI static guard drops btn-attachments-history from the
phone-hidden lock (it's opt-in now, excluded from the default-visible
enumeration — the guard still gates any NEW default-visible button); the
real-browser E2E now asserts default-hidden on a desktop-class viewport
and visible after enabling the setting.

Verified on a real desktop browser: hidden by default, the settings
toggle exists, enabling it shows the button. tsc + frontend-syntax +
prettier + public-asset checks + both test suites green.

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

src/web/public/index.html

@@ -114,7 +114,7 @@
           </div>
         </div>
         <button class="btn-icon-header btn-response-viewer-header btn-response-viewer-header--hidden" onclick="app.toggleResponseViewer()" title="View last response" aria-label="View last response"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z"/><circle cx="12" cy="12" r="3"/></svg></button>
-        <button class="btn-icon-header btn-attachments-history" id="attachmentsHistoryBtn" onclick="app.toggleAttachmentHistory()" title="Attachments" aria-label="Open attachment history" aria-expanded="false">
+        <button class="btn-icon-header btn-attachments-history btn-attachments-history--hidden" id="attachmentsHistoryBtn" onclick="app.toggleAttachmentHistory()" title="Attachments" aria-label="Open attachment history" aria-expanded="false">
           <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"/></svg>
           <span class="attachment-history-badge" id="attachmentHistoryBadge" style="display:none;">0</span>
         </button>
@@ -1016,6 +1016,13 @@
                   <span class="slider"></span>
                 </label>
               </div>
+              <div class="settings-item" title="Show the attachments button in header (opens the attachment history drawer)">
+                <span class="settings-item-label">Attachments Button</span>
+                <label class="switch switch-sm">
+                  <input type="checkbox" id="appSettingsShowAttachmentsButton">
+                  <span class="slider"></span>
+                </label>
+              </div>
               <div class="settings-item" title="Show the multi-monitor button in the header (opens Codeman spanned across all displays)">
                 <span class="settings-item-label">Multi-monitor Button</span>
                 <label class="switch switch-sm">