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.

[Marve10s]

Thanks for working on this. I reviewed the bug path and the issue is real: when Cursor model discovery rejects because the CLI is missing/unauthenticated/unavailable, the shared model picker can end up in the discovery error path instead of degrading Cursor only.

Please tighten this PR before merge:

1. Clean up the added explanatory comments. Keep only comments that are necessary for non-obvious behavior.
2. Keep the PR scoped to the model-discovery failure bug. Do not add unrelated edits, formatting churn, or extra test expansion beyond what is needed for this bug.
3. Apply the proper fix for the loading-state issue in the current patch. providerModelsQueryOptions uses placeholderData, and with TanStack Query the first unresolved fetch can report isLoading: false, isFetching: true, and isPlaceholderData: true. Changing Cursor/Kilo discovery pending from isLoading || isFetching to only isLoading means initial discovery may no longer show the loading state and can expose fallback static models while discovery is still running. Please distinguish the initial placeholder fetch from later background refetches, and add/adjust focused coverage for that behavior.

[Zeus-Deus]

Addressed all 3 points in #121. Loading state now uses isFetching && isPlaceholderData so the initial fetch is caught but background refetches aren't, plus a test for it.