perf: self-host fonts, eliminate CLS, a11y and contrast fixes

- Self-host Inter and JetBrains Mono (woff2, 69KB total)
- Remove Google Fonts dependency entirely (no external requests)
- Service Worker pre-caches font files
- Eliminate font-swap layout shifts
- WCAG contrast fix: disabled button uses solid color instead of opacity
- Footer link always underlined for distinguishability
- Translated aria-labels via data-i18n-aria
- Dimmed QR with "Bezahlt" stamp on paid invoices
- Hide wallet/address buttons when invoice is paid

sw.js

@@ -5,8 +5,10 @@ var ASSETS = [
   '/app.js',
   '/i18n.js',
   '/style.css',
-  '/lib/qrcode.min.js'
-  // xmr-crypto.bundle.js is lazy-loaded and runtime-cached
+  '/lib/qrcode.min.js',
+  '/fonts/inter-400.woff2',
+  '/fonts/jetbrains-400.woff2'
+  // xmr-crypto.bundle.js and jspdf.min.js are lazy-loaded and runtime-cached
 ];
 
 self.addEventListener('install', function (e) {