fix: align no-auth provider settings state

[SivanCola]

Summary

Follow-up to #4685. This keeps Settings consistent with the new no-auth provider semantics by separating key presence from provider selectability.

• Exposes requiresKey and configured on ProviderView so the frontend does not treat keySet as availability.
• Lets no-auth custom providers appear in Settings model pickers and refresh model lists without api_key_env.
• Avoids missing-key startup notices for loopback/private no-auth providers that keep a legacy api_key_env.
• Updates browser mock behavior, labels, and regression tests.

Testing

• git diff --check
• go test ./internal/boot -count=1
• cd desktop && go test . -count=1
• cd desktop/frontend && corepack pnpm exec tsx src/__tests__/provider-model-refresh.test.ts

Notes

cd desktop/frontend && corepack pnpm exec tsc --noEmit -p tsconfig.test.json is currently blocked in this local worktree because generated Wails bindings are absent: ../../wailsjs/go/main/App cannot be resolved. The follow-on drift-check error comes from that missing generated module.

[chatgpt-codex-connector]

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