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.html

@@ -601,7 +601,7 @@
     </section>
 
     <footer>
-      <p>Silent Send v0.3.0</p>
+      <p id="footerVersion">Silent Send</p>
     </footer>
   </div>