Add AI browser prompt api

[ScriptSmith]

No description provided.

[greptile-apps]

Greptile Summary

This PR wires up Chrome's on-device Prompt API as a browser/* model provider: a window-side bridge relays calls from the service worker (which can't access globalThis.LanguageModel), the SW intercepts /v1/models, /v1/responses, and /v1/chat/completions for browser/ models, and the setup wizard gains a BrowserAiCard that tracks download state. Tool use is polyfilled via system-prompt injection plus responseConstraint schema enforcement since the native tools option is not yet accepted by Chrome.

All previously flagged issues — role: \"tool\" message mapping, tool_calls field in chat completions, event.loaded fraction semantics, the silent no-op download callback, and the stale availability cache — have been addressed in this revision.

Confidence Score: 5/5

Safe to merge — only P2 findings remain; no runtime crashes or data-loss paths identified

All previously flagged P1 issues (tool message drops, tool_calls field, availability cache staleness, no-op download callback) are resolved. The two remaining findings are P2: an incorrect null-output serialization in tool results and a transient wizard-flash when browser AI is already downloaded, neither of which blocks core functionality.

ui/src/service-worker/browser-ai.ts (null output serialization at line 788) and ui/src/components/WasmSetup/WasmSetupGuard.tsx (availability initialisation timing)

Important Files Changed

Filename	Overview
ui/src/service-worker/browser-ai.ts	New 1400-line SW module implementing Responses and Chat Completions APIs over the Prompt API bridge; tool-call polyfill via responseConstraint is well-structured; null output serialization in inputToMessages produces wrong markup (P2)
ui/src/services/browser-ai/bridge.ts	Window-side bridge relaying Prompt API calls from the SW via MessageChannel; abort handling, session lifecycle, cumulative-vs-delta streaming heuristic, and download-progress forwarding all look correct; event.loaded documented as 0-1 fraction
ui/src/components/WasmSetup/WasmSetupGuard.tsx	Orchestrates browser AI state, bridge installation, download flow, and availability-cache invalidation; transient wizard flash possible when availability resolves after the model-list query (P2)
ui/src/components/WasmSetup/WasmSetup.tsx	Refactors BrowserAiProp into a bundled optional to eliminate the silent no-op fallback; BrowserAiCard correctly wired in WelcomeStep and ProvidersStep
ui/src/components/WasmSetup/BrowserAiCard.tsx	New UI card for browser AI state (supported/downloadable/downloading/ready/unavailable); progress bar uses downloadProgress * 100 consistently with the 0-1 bridge value
ui/src/service-worker/sw.ts	Adds BROWSER_AI_AVAILABILITY_CHANGED message handler and routes browser-AI requests through maybeHandleBrowserAi before the WASM gateway; /v1/models augmentation calls gateway exactly once
ui/src/services/browser-ai/availability.ts	UA-based model detection (Edge -> Phi-4-Mini, Chrome/Brave -> Gemini Nano) and availability polling; Edge guard runs before Chrome regex so Edge UA is not misclassified
ui/src/services/browser-ai/types.ts	Type declarations for the Prompt API; event.loaded documented as 0-1 fraction with spec link
ui/src/services/browser-ai/index.ts	Barrel re-exports for the browser-ai service module
ui/src/components/ModelPicker/model-utils.ts	Adds 'Browser AI' to PROVIDER_LABELS; trivial change
ui/src/pages/providers/shared.tsx	Adds sky-500 colour entry for the browser provider; trivial change

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
ui/src/service-worker/browser-ai.ts:788-790
**Null tool output serialized as `""` not as empty string**

`JSON.stringify(item.output ?? "")` handles a `null`/`undefined` `output` field by substituting `""`, then JSON-encodes it to the literal `"\"\""` (two double-quote characters). The injected markup becomes `<tool_result name="tool">""</tool_result>`, which the model reads as the string `""` rather than an empty result. A `null` output should produce an empty tag body.

```suggestion
      const output =
        typeof item.output === "string"
          ? item.output
          : item.output != null
            ? JSON.stringify(item.output)
            : "";
```

### Issue 2 of 2
ui/src/components/WasmSetup/WasmSetupGuard.tsx:344-374
**Transient wizard flash when Browser AI is already downloaded**

`browserAi` state is initialised with `availability: "unavailable"` regardless of the actual on-device state. If the model list query settles before the `getAvailability()` promise (which talks to the bridge), `needsOnboarding` will briefly be `true` — opening the wizard — even when the browser AI model is already ready. Once `getAvailability()` resolves, `browserAiCounts` becomes true and the modal closes, but the flash is visible.

Consider initialising `availability` via a synchronous check if the API exposes one, or at minimum ensure `isLoading` stays true until both the model-list query and the availability check have settled.

_{Reviews (3): Last reviewed commit: "Review fixes" | Re-trigger Greptile}

[ScriptSmith]

@greptile-apps

[ScriptSmith]

@greptile-apps