Fix footer version, improve PDF filter detection

- options.html/js: Footer version now reads dynamically from manifest via
  api.runtime.getManifest().version instead of hardcoded v0.3.0
- document-scanner.js: Increase FlateDecode lookback from 200 to 500 bytes
  (real PDF stream dictionaries are often larger than 200 bytes), and support
  array form /Filter [/FlateDecode] alongside the scalar form

https://claude.ai/code/session_01ReZUeR1nrYzJeX7fTqwXMw

src/options/options.js

@@ -28,6 +28,9 @@ document.addEventListener('DOMContentLoaded', async () => {
   $('#maxLogEntries').value = settings.maxLogEntries || 200;
   $('#browserSync').checked = settings.browserSync === true;
 
+  const { version } = api.runtime.getManifest();
+  $('#footerVersion').textContent = `Silent Send v${version}`;
+
   renderMappings();
   renderPasswords();
   renderDomains();