chore: pin the built-in default model to deepseek/deepseek-v4-pro

[yujiezhang-ops]

Advances the pinned built-in default model from deepseek/deepseek-v3 to deepseek/deepseek-v4-pro. Groundwork for #76, which asks that a PPIO or Novita user only supply a key — pre-filling a model that is four releases behind would make the staleness visible to every new user on their first screen.

Why this value is load-bearing

Despite the field name, fallback_probe_model is not probe-only. internal/app/provider.go:260 (resolveProviderModel) and internal/app/install.go:187 both fall back to it, so it is the model written into a real Agent config whenever the user does not pick one.

v4 removes a trap

PPIO published v3 as deepseek/deepseek-v3 and Novita as deepseek/deepseek_v3 — one hyphen versus one underscore, so the two entries could never share a constant. Both now publish deepseek/deepseek-v4-pro. Verified present in each /v1/models listing (PPIO 108 models, Novita 145; both answer anonymously).

This makes #76's acceptance criterion "assert the two defaults are distinct strings" obsolete; that issue has been updated.

Why v4-pro

v4-flash-0731 has the newest created timestamp but is a dated snapshot — pinning it means going stale the moment the next snapshot ships. v4-flash and v4-pro are both rolling aliases from the same date.

Chose pro over flash because this value reaches real configs, where coding capability outweighs the per-token saving. The gap is worth knowing: on PPIO pro costs 3× flash (30000/60000 vs 10000/20000 per M), on Novita 11× (16000/32000 vs 1400/2800). If first-run cost matters more than capability, switching to v4-flash is three lines in the lock file.

Scope

providers.lock.json — default_fallback_probe_model plus both providers' fallback_probe_model.

The i18n key 例如 deepseek/deepseek-v3 and its three call sites (ProviderKeyPage.tsx:133, ProfilesPage.tsx:260, AgentProfilePage.tsx:214). i18n is Chinese-source, so the key itself moves.

ModelPicker.tsx:52 keeps its neutral 例如 gpt-4.1 — that is the free-text box, where a vendor-neutral example is more appropriate than promoting one provider's model.

Verification

go test ./... — all packages pass. Note internal/catalog/catalog_test.go:68 asserts the public provider projection contains no "deepseek" substring; that still holds because PublicProviders zeroes fallbackModel.

pnpm run test — 26 files, 205 tests pass. pnpm run build (which runs tsc --noEmit first) succeeds.

Also confirmed upstream that deepseek/deepseek-v4-pro is a live model id on both providers, not just newer-looking.

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