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/manifest.json

@@ -0,0 +1,14 @@
+{
+  "name": "Qrcode Generator",
+  "short_name": "QRcode",
+  "description": "Self-hosted static and dynamic qr code generator",
+  "start_url": "index.php",
+  "scope": "./",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#007bff",
+  "icons": [
+    { "src": "dist/img/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "dist/img/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}