feat: add desktop/mobile device toggle (User-Agent)

- Segmented Desktop / Mobile button in the form
- Desktop: Chrome 124 on Windows
- Mobile: iPhone Safari 17 (iOS 17.4)
- User-Agent injected into k6 script headers
- Stored in DB with migration for existing data
- History items show device pill alongside cache/gzip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/style.css

@@ -152,6 +152,30 @@ textarea { font-family: var(--mono); resize: vertical; }
 .pill.bust     { background: rgba(239,68,68,.12);  color: #f87171; }
 .pill.gzip     { background: rgba(124,58,237,.15); color: var(--accent2); }
 .pill.no-gzip  { background: rgba(100,116,139,.12); color: var(--muted); }
+.pill.desktop  { background: rgba(56,189,248,.12);  color: #38bdf8; }
+.pill.mobile   { background: rgba(251,146,60,.12);  color: #fb923c; }
+
+/* DEVICE TOGGLE */
+.device-toggle { display: flex; gap: 0; margin-top: .15rem; }
+.device-toggle input[type=radio] { display: none; }
+.device-btn {
+  flex: 1;
+  text-align: center;
+  padding: .45rem .75rem;
+  font-size: .85rem;
+  border: 1px solid var(--border);
+  cursor: pointer;
+  transition: background .15s, color .15s;
+  color: var(--muted);
+  background: transparent;
+}
+.device-btn:first-of-type { border-radius: 6px 0 0 6px; }
+.device-btn:last-of-type  { border-radius: 0 6px 6px 0; border-left: none; }
+.device-toggle input[type=radio]:checked + .device-btn {
+  background: var(--accent);
+  color: #fff;
+  border-color: var(--accent);
+}
 .btn-secondary:hover { background: rgba(124,58,237,.15); }
 
 /* RESULT */