feat(ui): add header display settings for font controls, system stats, and token count

- Add visibility toggles in App Settings modal for A-/A+, CPU/MEM, and token count
- Make settings gear icon larger (32px) for better accessibility
- Apply settings on startup and immediately when saved
- All options enabled by default, persisted to localStorage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/public/styles.css

@@ -284,6 +284,13 @@ body {
   transform: rotate(45deg);
 }
 
+.btn-icon-header.btn-settings {
+  width: 32px;
+  height: 32px;
+  font-size: 1.4rem;
+  margin-left: 0.5rem;
+}
+
 .btn-icon-header.btn-sm {
   width: 20px;
   height: 20px;
@@ -1347,6 +1354,27 @@ body {
   background: #dc3545;
 }
 
+.settings-section {
+  margin-bottom: 1.25rem;
+  padding-bottom: 1rem;
+  border-bottom: 1px solid var(--border);
+}
+
+.settings-section:last-child {
+  margin-bottom: 0;
+  padding-bottom: 0;
+  border-bottom: none;
+}
+
+.settings-section-title {
+  font-size: 0.75rem;
+  font-weight: 600;
+  color: var(--text);
+  margin-bottom: 0.75rem;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
 .form-row {
   margin-bottom: 0.75rem;
 }