fix(translation): use consistent HTTP transport for AI requests

[marcomarcogd]

Summary

• construct AI clients from a cloned default transport with HTTP/2 negotiation and HTTP/1.1 fallback
• apply the same proxy, TLS, authentication, endpoint and custom-header path to profile tests and actual AI features
• preserve the injected HTTP client when translation or summary prompts and headers change
• make the translation factory use the proxy-aware AI translator

Fixes #1044

Related to #918: that report does not include enough reproduction data to close automatically, but users seeing the same AI transport failure can retest this build.

Validation

• target Go tests for translation, HTTP utilities, AI handlers, chat, service and summary
• actual HTTP/2 gateway translation test
• HTTP/1.1 fallback test
• proxy and custom-header preservation tests
• git diff --check