Fix model picker breaking when Cursor CLI is unavailable

[Zeus-Deus]

Fixes #103

When the Cursor CLI isn't installed or authenticated, Cursor's model discovery fails — and that failure was rejecting the whole models query, leaving the model picker stuck in a loading/error state for every provider.

Changes

• providerModelsQueryOptions now catches discovery failures and resolves to an empty result instead of rejecting. A failing CLI degrades only that provider; every other provider keeps its models.
• The Cursor/Kilo discovery skeleton is gated on isLoading instead of isFetching, so background refetches don't re-blank the picker once discovery has settled.
• Added tests covering a single provider failing without affecting the others, plus a missing kilo key in an existing test fixture.

Testing

• bun lint and bun run test pass.
• Verified in the app: with Cursor's binary path pointed at a missing binary, the picker opens normally, Cursor falls back to its static model list, and every other provider works as usual.

Note: bun typecheck still reports one pre-existing, unrelated error in composerDraftStore.ts (Pi provider), and two server tests fail for environment reasons — both already present on main and untouched here.