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

README.md

@@ -1,6 +1,16 @@
 # Silent Send
 
-A browser extension (Chrome + Firefox) that intercepts personal information and substitutes it with user-defined replacements before sending to Claude.ai.
+A browser extension (Chrome + Firefox) that intercepts personal information and substitutes it with user-defined replacements before sending to AI services.
+
+### Supported services
+
+| Service | Domains |
+|---------|---------|
+| Claude | claude.ai, claude.ai/code |
+| ChatGPT | chatgpt.com, chat.openai.com |
+| Grok | grok.x.ai, x.com/i/grok |
+| Gemini | gemini.google.com |
+| OpenWebUI | localhost, 127.0.0.1 (self-hosted) |
 
 ## How it works