Settings gear in topbar linking to placeholder /settings page (#36)

- layout.html: inline SVG gear icon top-right, styled as .iconbtn.gear pill
- new settings.html template reusing standard layout with under-construction card
- register GET /settings route in main.go

web/static/app.css

@@ -128,6 +128,11 @@ body {
 .stats-v { color: var(--fg); word-break: break-all; }
 .meta-bar .spacer { flex: 1; }
 .iconbtn { border: 1px solid var(--border); background: var(--surface-2); color: var(--muted-fg); border-radius: var(--radius); padding: 5px 12px; font: inherit; font-size: 21.6px; cursor: pointer; text-decoration: none; }
+.iconbtn.gear { display: inline-flex; align-items: center; padding: 5px 9px; }
+.iconbtn.gear svg { width: 22px; height: 22px; }
+.settings-head { padding: 12px 18px; border-bottom: 1px solid var(--border); }
+.settings-head h1 { font-size: 29.2px; font-weight: 600; margin: 0; }
+.settings-body { padding: 16px 18px; color: var(--muted-fg); font-size: 21.6px; }
 .iconbtn:hover { color: var(--fg); border-color: var(--muted); }
 .iconbtn.danger:hover { color: #ff8fa3; border-color: #ff8fa3; }
 .code-head {