Web: settings page nav/header follow the surface colour too

The settings modal's own left nav and header were on --bg-app; point them at
--bg-sidebar / --bg-topbar so they track surface_color like the main sidebar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/css/app.css

@@ -1280,6 +1280,7 @@ a { color: var(--primary); text-decoration: none; }
   display: flex; align-items: center; gap: 8px;
   padding: 14px 20px; border-bottom: 1px solid var(--border);
   flex-shrink: 0;
+  background: var(--bg-topbar);
 }
 .settings-page-header h3 { font-size: 16px; font-weight: 600; color: var(--text-1); margin: 0; }
 .settings-page-body {
@@ -1290,6 +1291,7 @@ a { color: var(--primary); text-decoration: none; }
   border-right: 1px solid var(--border);
   padding: 12px 8px;
   display: flex; flex-direction: column; gap: 2px;
+  background: var(--bg-sidebar);
 }
 .settings-nav-btn {
   display: block; width: 100%; text-align: left;

frontend/sw.js

@@ -7,7 +7,7 @@
 // the entry HTML / version files). New releases take effect on the next
 // reload, no manual SW unregister required.
 
-const CACHE_VERSION  = 'calendarr-v28';
+const CACHE_VERSION  = 'calendarr-v29';
 const OFFLINE_SHELL  = ['/', '/index.html'];
 
 self.addEventListener('install', event => {