Wire up document scanner for file upload interception, bump to 0.9.23

- Inject document-scanner.js into page world via injector.js (as module,
  sets globalThis.DocumentScanner)
- Add FormData interception to fetch hook: scans File/Blob entries through
  DocumentScanner.processUpload(), also substitutes string fields
- Add document-scanner.js to web_accessible_resources in both manifests
- Bump version to 0.9.23 in package.json, manifest.json, manifest.firefox.json

https://claude.ai/code/session_01NNBEPuXMFGWezJb1f958nL

package.json

@@ -1,6 +1,6 @@
 {
   "name": "silent-send",
-  "version": "0.9.21",
+  "version": "0.9.23",
   "private": true,
   "license": "MIT",
   "description": "Browser extension that substitutes personal data before sending to AI services",