feat(ui): show model count as enabled/total

[245678000000]

Closes the first half of #9.

Summary

Both counters in the sidebar showed only the total, so there was no way to tell how many of a provider's models are actually registered into Cursor's model picker without expanding the provider and reading every toggle.

Where	Was	Now
Provider accordion head	7 models	2/7 models
Models section title	Models (7)	Models (2/7)

Both spans also get a title="Enabled / total models" so the ratio explains itself on hover.

Notes

• Counting lives in a new store helper modelCountLabel(), following getModelErrors()'s precedent of keeping long expressions out of the templates.
• "Enabled" is defaultOn === true — the same test the model-card toggle binds to (x-bind:checked="m.defaultOn === true"), so the ratio always matches what the switches show.
• The accordion head loses its 1 model singular form, which no longer reads sensibly next to a ratio.
• The second half of enhancement. 关于两个页面显示功能 #9 (editing apiModel/displayName on an existing model sometimes not refreshing in chat until the server is restarted) is not addressed here — that one needs digging into the client-side cache and deserves its own PR.

Test plan

The repo has no UI test harness (vitest only collects src/server/tests/**), so instead of restructuring that for one label, I rendered the two components through their hono/jsx factory and evaluated the emitted x-text against the helper:

acc-meta     x-text: $store.app.modelCountLabel(p.id) + ' models'
models-title x-text: 'Models (' + $store.app.modelCountLabel(p.id) + ')'

  none configured          -> "0/0 models"   "Models (0/0)"
  1 of 1 on                -> "1/1 models"   "Models (1/1)"
  0 of 1 (flag missing)    -> "0/1 models"   "Models (0/1)"
  2 of 5                   -> "2/5 models"   "Models (2/5)"

• pnpm --dir "Cursor++" run check-types
• pnpm --dir "Cursor++" run lint
• node esbuild.js --production
• Not exercised in a running Cursor sidebar — worth a quick look before merging.