feat(provider): add Atlas Cloud support - #16

Upstream's 'unify model providers and migrate AICON gateway' refactor replaced
almost everything this branch touched, so most of the PR is now deleted rather
than merged. Net diff against main drops from 8 files to 3.

What upstream changed:
- ProviderFactory's per-provider match statement became one GatewayProvider
  plus registry.connection(), so the 'case "atlascloud"' branch has no home.
- The hardcoded frontend provider list was removed in favour of a backend
  /api-keys/provider-presets endpoint that serves registry.PROVIDERS verbatim.
- APIKeys.vue / APIKeysSettings.vue gained applyProviderPreset, which does
  exactly what this branch's handleProviderChange did, but driven by that
  endpoint.
- validate_provider now derives its list from PROVIDERS, and get_models now
  reads the catalog instead of hardcoding per-provider model names.

So the whole change is one registry entry:

    'atlascloud': ('Atlas Cloud', 'https://api.atlascloud.ai/v1'),

Two things the merge would otherwise have shipped quietly:
- APIKeys.vue and APIKeysSettings.vue auto-merged into a select carrying two
  @change attributes, with this branch's handler calling
  apiKeyUtils.getProviderDefaultBaseUrl — a function upstream deleted. Both
  handlers and the duplicate attribute are removed; upstream's preset path
  stands alone.
- The contract test asserted ProviderFactory.create returns a CustomProvider,
  which is no longer true. It is rewritten against the new architecture and now
  covers the registry preset, connection() resolution for both default and
  explicit base URLs, rejection of credentialed/queried/non-HTTP base URLs, and
  acceptance by the schema validator and the provider enum.

Verified locally: the four tests pass, and the presets endpoint returns the
Atlas Cloud row alongside the six existing providers. Note libmagic is not
installed on this machine, so the test had to be run with the app's optional
python-magic import stubbed; the repository has no CI workflow to fall back
on.