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

api/shorten.php

@@ -0,0 +1,61 @@
+<?php
+header('Content-Type: application/json');
+header('Access-Control-Allow-Origin: *');
+header('Access-Control-Allow-Methods: POST');
+header('Access-Control-Allow-Headers: Content-Type');
+
+if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') { exit; }
+if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+    http_response_code(405);
+    echo json_encode(['error' => 'Method not allowed']);
+    exit;
+}
+
+$dataDir = __DIR__ . '/../data';
+$dbFile = $dataDir . '/urls.json';
+
+if (!is_dir($dataDir)) {
+    mkdir($dataDir, 0750, true);
+}
+
+$input = json_decode(file_get_contents('php://input'), true);
+$hash = $input['hash'] ?? '';
+
+if (empty($hash) || strlen($hash) > 500) {
+    http_response_code(400);
+    echo json_encode(['error' => 'Invalid data']);
+    exit;
+}
+
+// Load existing URLs
+$urls = [];
+if (file_exists($dbFile)) {
+    $urls = json_decode(file_get_contents($dbFile), true) ?: [];
+}
+
+// Check if this hash already exists
+$existing = array_search($hash, $urls);
+if ($existing !== false) {
+    echo json_encode(['code' => $existing]);
+    exit;
+}
+
+// Generate short code (6 chars)
+function generateCode($length = 6) {
+    $chars = 'abcdefghijkmnpqrstuvwxyz23456789';
+    $code = '';
+    for ($i = 0; $i < $length; $i++) {
+        $code .= $chars[random_int(0, strlen($chars) - 1)];
+    }
+    return $code;
+}
+
+$code = generateCode();
+while (isset($urls[$code])) {
+    $code = generateCode();
+}
+
+$urls[$code] = $hash;
+file_put_contents($dbFile, json_encode($urls, JSON_UNESCAPED_UNICODE), LOCK_EX);
+
+echo json_encode(['code' => $code]);