fix: Firefox signing — manifest_version corruption + rate limiting

The sign script's sed command was replacing ALL "version" patterns
including "manifest_version": 3, corrupting it to "manifest_version": 1.
Fixed by anchoring the sed pattern to only match the top-level
"version" field (starts with two spaces at line beginning).

Also:
- Added data_collection_permissions to Firefox manifest (new Mozilla
  requirement for all extensions)
- Added 8-second sleep between retry attempts to avoid Mozilla API
  rate limiting (was causing cascading failures)

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

manifest.firefox.json

@@ -6,7 +6,12 @@
   "browser_specific_settings": {
     "gecko": {
       "id": "silent-send@example.com",
-      "strict_min_version": "128.0"
+      "strict_min_version": "128.0",
+      "data_collection_permissions": {
+        "required": false,
+        "optional": false,
+        "description": "Silent Send does not collect, transmit, or store any data externally. All processing is 100% local in your browser."
+      }
     }
   },
   "permissions": [