feat: complete v1 — QR invoice generator with i18n, short URLs, offline support

- XMR address validation (standard, subaddress, integrated)
- Amount in XMR/EUR/USD/CHF with CoinGecko conversion
- QR code generation with monero: URI
- Shareable short URLs (/s/abc123) via self-hosted PHP backend
- i18n (DE/EN) with browser language detection
- Service worker for offline capability
- Dark mode, responsive design

sw.js

@@ -0,0 +1,58 @@
+var CACHE_NAME = 'xmrpay-v1';
+var ASSETS = [
+  '/',
+  '/index.html',
+  '/app.js',
+  '/i18n.js',
+  '/style.css',
+  '/lib/qrcode.min.js'
+];
+
+self.addEventListener('install', function (e) {
+  e.waitUntil(
+    caches.open(CACHE_NAME).then(function (cache) {
+      return cache.addAll(ASSETS);
+    })
+  );
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', function (e) {
+  e.waitUntil(
+    caches.keys().then(function (names) {
+      return Promise.all(
+        names.filter(function (n) { return n !== CACHE_NAME; })
+             .map(function (n) { return caches.delete(n); })
+      );
+    })
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', function (e) {
+  var url = new URL(e.request.url);
+
+  // CoinGecko API — network only, don't cache
+  if (url.hostname !== location.hostname) {
+    e.respondWith(fetch(e.request));
+    return;
+  }
+
+  // App assets — cache first, fallback to network
+  e.respondWith(
+    caches.match(e.request).then(function (cached) {
+      var networkFetch = fetch(e.request).then(function (response) {
+        if (response.ok) {
+          var clone = response.clone();
+          caches.open(CACHE_NAME).then(function (cache) {
+            cache.put(e.request, clone);
+          });
+        }
+        return response;
+      }).catch(function () {
+        return cached;
+      });
+      return cached || networkFetch;
+    })
+  );
+});