Fase 3 priority 2: presets, random style, qr scanner, PWA

Preset system: qr_presets table (migration 005) plus a presets.php AJAX
endpoint (list/save/delete, CSRF-protected, scoped to the logged-in user's
own id - presets are personal, never shared across accounts). UI/JS lives in
dist/js/qrcode-style-tools.js.

Random style button: client-side only, fills foreground/background with a
random hex color pair (playful randomize, no contrast/scannability
guarantee).

Qr scanner (scan_qrcode.php): camera or image upload, decoded entirely
client-side via html5-qrcode (CDN, pinned to 2.3.8).

PWA: manifest.json + service-worker.js, icons generated from the existing
DynamicQRCode_Original.png glyph. The service worker only caches static
assets (css/js/images) and deliberately never touches PHP pages, since those
carry CSRF tokens and session-specific content that must never be cached.

Fixes a gap found while testing: qrcode_options.php is only a shared partial
for the static qr forms - the dynamic qr form (form_dynamic_add.php) has its
own separate copy of the foreground/background/level/size/filename/format
fields (pre-existing structure, not something introduced here). That meant
frame_text and the new preset/random-style UI never showed up on the
dynamic qr page. Added the same fields there too, verified with a dynamic qr
plus frame text (150x180px, the expected +30px padding).

src/includes/sidebar.php

@@ -32,6 +32,14 @@
                Dashboard
              </p>
            </a>
+         </li>
+         <li class="nav-item">
+           <a href="./scan_qrcode.php" <?php echo (CURRENT_PAGE == 'scan_qrcode.php') ? ' class="nav-link active"' : ' class="nav-link"'; ?>>
+             <i class="nav-icon fas fa-camera"></i>
+             <p>
+               Scan qr code
+             </p>
+           </a>
          </li>
           <?php if ($_SESSION['type'] !== 'user' || !empty($_SESSION['can_view_dynamic'])): ?>
           <li <?php echo ((substr(CURRENT_PAGE, 0, 19) == 'dynamic_qrcodes.php') || (substr(CURRENT_PAGE, 0, 18) == 'dynamic_qrcode.php')) ? ' class="nav-item has-treeview menu-open"' : ' class="nav-item has-treeview"'; ?>>