feat(concurrency): provider dropdown instead of free-text input

Replace the Add-form provider text input with a <select> dropdown populated
from the available models' provider prefixes (<provider>/<model>), unioned
with providers already carrying a configured limit (so a limit set out-of-band
but whose model list is empty still appears). The selection auto-defaults to
the first option and falls back when an option disappears.

- logic/view-model.ts: pure providerFromModel + providerOptions(models, limits)
  (distinct provider ids, first-seen order) + 7 tests.
- ConcurrencyView.svelte: models prop + <select> bound to newProviderId;
  disabled + 'No providers available' when there are no models.
- App.svelte: pass models={store.models}; component test updated to the
  dropdown (selectOptions) + a no-models case (6 component tests).

Verified: typecheck 0/0, 922 tests green, biome clean, build OK.

0 files changed, 0 insertions, 0 deletions