feat: add support for ChatGPT, Grok, Gemini, and OpenWebUI

Extend Silent Send to intercept API requests on all major AI chat
services. Each service has different API shapes:

- ChatGPT: /backend-api/conversation with content.parts arrays
- Grok: GraphQL + /2/grok/add_response with message field
- Gemini: form-encoded f.req with nested arrays (+ generateContent)
- OpenWebUI: /api/chat and /ollama/api/chat (self-hosted)

All services share the same substitution pipeline. Manifests updated
for both Chrome and Firefox with host_permissions for all domains.
OpenWebUI supported via localhost/127.0.0.1 for self-hosted instances.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/popup/popup.html

@@ -142,7 +142,7 @@
     <!-- Test Tab -->
     <section class="tab-content" id="tab-test">
       <p class="help-text">Type text containing your real values to see what would be sent.</p>
-      <textarea id="testInput" class="textarea" placeholder="Try typing: My name is John Smith and my email is john@example.com" rows="4"></textarea>
+      <textarea id="testInput" class="textarea" placeholder="Try: My name is John Smith, email john@gmail.com, logged in as jsmith@macbook-pro" rows="4"></textarea>
       <div class="diff-view" id="diffView">
         <div class="diff-label">What gets sent:</div>
         <div class="diff-output" id="diffOutput"></div>