fix: camera button (📷) intercepted by kiosk native btnSettings overlay

- Kiosk (Android): btnSettings was positioned top|end with alpha=0.12,
  sitting invisibly on top of the HTML scan button in the webapp header.
  Moved to bottom|end (marginBottom=80dp, alpha=0.28) so it never
  overlaps the header. Kiosk versionCode 15→16, versionName 1.7.15.
- Web (Android Chrome/Brave): pointerleave fired before pointerup when
  finger drifted, cancelling the long-press timer and letting a synthetic
  click bubble to an unintended handler. Fixed with setPointerCapture +
  preventDefault + replaced pointerleave with pointercancel. Added
  touch-action:manipulation to .header-scan-btn CSS.

assets/css/style.css

@@ -279,6 +279,7 @@ body {
     height: 48px;
     box-shadow: 0 2px 8px rgba(0,0,0,0.18);
     animation: pulse-scan 2s ease-in-out infinite;
+    touch-action: manipulation; /* prevent 300ms delay and double-tap zoom on mobile */
 }
 .header-scan-btn:active {
     background: rgba(255,255,255,0.45);