fix: config not reaching content.js — race condition with script removal

The data-ss-config attribute on the script tag was being removed
(via script.onload) before content.js could read it. Changed approach:
inject config as a separate <script type="application/json" id="ss-config-data">
element that persists in the DOM until content.js reads and removes it.

This eliminates the race condition between script execution and onload
removal. Bump 0.9.20.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.19",
+  "version": "0.9.20",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {