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

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.21",
+  "version": "0.9.23",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {
@@ -114,7 +114,7 @@
   ],
   "web_accessible_resources": [
     {
-      "resources": ["src/content/content.js", "src/content/early-hook.js"],
+      "resources": ["src/content/content.js", "src/content/early-hook.js", "src/lib/document-scanner.js"],
       "matches": ["<all_urls>"]
     }
   ]