Security hardening: rate limiting, atomic locks, origin check, honest docs

API / Security:
- Add api/_helpers.php: shared send_security_headers(), verify_origin(),
  get_hmac_secret(), check_rate_limit(), read_json_locked(), write_json_locked()
- shorten.php: remove Access-Control-Allow-Origin:*, restrict to same-origin,
  rate-limit 20 req/h per IP, atomic JSON read+lock, HMAC secret from file
- verify.php: rate-limit GET (30/min) and POST (10/h) per IP, atomic lock,
  prevent overwriting existing proofs, origin check on POST
- node.php: fix rate limit from 1000 to 60 req/min, add security headers,
  origin check
- check-short.php: add security headers, re-derive signature server-side
- s.php: use file-based HMAC secret via get_hmac_secret(), hash_equals()
  for timing-safe comparison

Service Worker:
- sw.js: navigation requests (mode=navigate) never served from cache;
  network-first with offline fallback to prevent stale invoice state

Documentation (honest claims):
- README: tagline "No backend" -> "No tracking"; new Architecture table
  listing exactly what server sees for each feature; Security Model section
- index.html: meta description and footer updated from "No Backend" to
  "Minimal Backend"
- i18n.js footer: already updated in previous commit

sw.js

@@ -41,6 +41,17 @@ self.addEventListener('fetch', function (e) {
     return;
   }
 
+  // Navigation (HTML) — network first, fall back to cached index.html for offline
+  // Invoice data is in the URL hash, so caching the document would cause stale state
+  if (e.request.mode === 'navigate') {
+    e.respondWith(
+      fetch(e.request).catch(function () {
+        return caches.match('/index.html');
+      })
+    );
+    return;
+  }
+
   // App assets — cache first, fallback to network
   e.respondWith(
     caches.match(e.request).then(function (cached) {