refactor(desktop): split onboarding overlay god file into onboarding/ folder

desktop-onboarding-overlay.tsx (1,291 lines) folded into components/onboarding/
as a cohesive feature folder. Behaviour-preserving — every move is verbatim;
typecheck + lint + tests green.

- index.tsx (665) — overlay shell, Picker, Header/Preparing, API-key catalog +
  ApiKeyForm; re-exports the provider API the settings page consumes.
- flow.tsx (364) — OAuth flow panels (FlowPanel, steps, DeviceCode, CodeBlock,
  ConfirmingModelPanel, DocsLink, Status).
- providers.tsx (118) — provider rows + display/sort (FeaturedProviderRow,
  ProviderRow, KeyProviderRow, providerTitle, sortProviders).
- glyph.tsx (170) — the decode/scramble animation toolkit (pure leaf).

Importers (desktop-controller, providers-settings) repointed to
@/components/onboarding; the overlay test moved to onboarding/index.test.tsx.

apps/desktop/src/app/settings/providers-settings.tsx

@@ -10,7 +10,7 @@ import {
   ProviderRow,
   providerTitle,
   sortProviders
-} from '@/components/desktop-onboarding-overlay'
+} from '@/components/onboarding'
 import { Button } from '@/components/ui/button'
 import { RowButton } from '@/components/ui/row-button'
 import { SearchField } from '@/components/ui/search-field'