feat: allow custom model User-Agent overrides

[hanzheng1954]

Summary

• add an optional per-custom-model user_agent setting
• persist it in SQLite and expose it through custom model CRUD/test APIs
• apply the override through the shared LLM HTTP transport
• add React and Vue model-form fields
• preserve Shelley's default Shelley/<commit> User-Agent when unset

Motivation

Some OpenAI-compatible providers restrict channels by client User-Agent. Custom models currently cannot use those providers because Shelley always overwrites User-Agent.

Validation

• go test ./llm/llmhttp ./db ./models ./server
• pnpm run type-check
• pnpm run lint
• pnpm run build
• live test through /api/custom-models-test: default Shelley UA was rejected with 403; configured custom UA succeeded

Full go test ./... still has pre-existing unrelated failures on current main in Anthropic metadata/tool-schema tests, plus templates must be generated before cmd/templates compilation.

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[philz]

Thanks for the contribution. This currently seems pretty niche to me. I'd recommend you run a proxy locally instead. If you let me know what providers you're running into issues with, happy to reconsider.