Back/forward buttons: adjustable opacity; Upgrade refreshes the Web UI copy

New historyButtonsOpacity setting (10-100 percent, default 50) in the
terminal menu (Display & Interaction) and the Web UI. Upgrade now also
copies the Web UI app to /opt/kiosk-webui and restarts it, so new Web UI
settings appear after an upgrade (it previously kept its old copy).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wi3ZQveYdjfcrEsyazq32

webui/public/index.html

@@ -105,6 +105,10 @@
                 <option value="-1">Disabled</option>
               </select>
             </label>
+            <label>
+              Back/forward button opacity (10 = faint, 100 = solid)
+              <input type="number" name="historyButtonsOpacity" min="10" max="100" step="5">
+            </label>
             <label>
               Inactivity timeout (minutes)
               <input type="number" name="inactivityTimeoutMinutes" min="1" max="240" step="1">