fix: ChatGPT fetch interception failure + bump to 2.0.6

ChatGPT (and potentially other AI services) calls fetch() with a
Request object as the first argument: fetch(new Request(url, opts))
instead of fetch(url, opts). The interceptor only handled the second
form, so ChatGPT's conversation requests passed through unmodified.

Fixed by handling both fetch signatures:
- fetch(url, options) — existing path
- fetch(Request) — new: extracts URL, method, headers, reads body
  via request.text() for JSON/text content types

Also handles non-string body types:
- Blob → text via blob.text()
- ArrayBuffer → text via TextDecoder
- URLSearchParams → string via toString()

These cover the various ways modern frameworks call fetch().

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "2.0.5",
+  "version": "2.0.6",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {