[settings] 用户无法配置自己的 AI API，AI 请求仍固定使用 Workers AI

[jamiesun]

Context Boundary

• Repo/module: talkincode/GameKit, AI controls in the browser UI and /api/ai Worker
• Version/commit: main, 9eb811e7bc3db3193b57609425723c478e438d32
• Runtime: browser application calling a Cloudflare Worker
• OS/browser: not recorded; provider-specific browser/CORS behavior is not yet selected
• Dependencies/config: current text and image models are Workers AI bindings; no provider setting exists

Expectation vs. Reality

Expected: Users can configure their own AI API credentials and provider/model in Settings, then choose that configuration for AI requests.

Actual: The client sends text and image requests to /api/ai, while the Worker uses fixed Workers AI model constants. There is no UI or stored configuration for a user-owned endpoint, API key, or model.

Reproduction Path

1. Open GameKit and choose an AI action.
2. Try to select a different provider or enter a personal API endpoint, key, and model.
3. Observe that no such setting is available and requests continue through /api/ai to the fixed Worker models.

Minimal reproduction: the client call targets /api/ai; the Worker selects TEXT_MODEL or IMAGE_MODEL without reading user provider configuration.

Blast Radius

• Affected users/paths: users who want to use their own provider, model, or billing account.
• Frequency: every AI text or image request currently uses the configured Workers AI binding.
• Severity rationale: users cannot choose their own provider; if the goal is to stop spending the operator's public Workers AI quota, recommend P1 for the product decision, otherwise P2 as a provider-choice enhancement.
• Workaround: use the current built-in provider or generate content in an external tool and import it manually.

Evidence

• Client requests go through /api/ai: store.tsx, asset request
• Worker models are fixed: worker/index.ts, model calls
• The public quota boundary is documented: runtime.md

Decisions Needed

• Should user-owned APIs be an additional option, or replace the built-in Workers AI path?
• Which first-version contract is intended (for example, OpenAI-compatible text API), and is image generation in scope?
• API keys must not enter project files/exports. The same-origin preview isolation finding in issue #6 should be resolved before persisting credentials in browser storage.