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/index.html

@@ -67,6 +67,17 @@
             </select>
           </div>
 
+          <div class="form-group">
+            <label for="device">Device</label>
+            <div class="device-toggle">
+              <input type="radio" name="device" id="device-desktop" value="desktop" checked />
+              <label for="device-desktop" class="device-btn">&#x1F5A5; Desktop</label>
+              <input type="radio" name="device" id="device-mobile" value="mobile" />
+              <label for="device-mobile" class="device-btn">&#128241; Mobile</label>
+            </div>
+            <div class="hint" id="ua-hint">Chrome 124 on Windows</div>
+          </div>
+
           <div class="form-group">
             <label>Encoding</label>
             <div class="toggle-row">