- Fix biome formatting (indentation) across all changed files
- Remove unused variable 'displayNameField' in ModelForm.tsx
- Remove unused import 'formatProviderLabel' in ModelForm.tsx
- Remove unused destructured 'label' in ProviderForm.tsx
- Add DeleteDialog confirmation to ProviderForm delete action
- Wire up model deletion from ModelForm to ModelsSection's DeleteDialog
- Fix ProvidersSection to fall back to list view when provider disappears
- Replace template string classNames with cn() utility in ProvidersSection
  and ModelsSection for consistency

- ModelConfigFields: inline renderProviderSpecificField (only called once)
- ModelForm: remove empty try/catch block, replace div role='button' with
  actual button elements, use formik getFieldProps for displayName
- ModelsSection: simplify ternary to function with if statements, use
  Tailwind divide-y utility instead of conditional borders, replace
  div role='button' with button element
- ProviderForm: replace useMemo with useState lazy initializer for
  initialValues, remove useEffect sync pattern (parent should handle
  re-keying), replace div role='button' with button element
- ProvidersSection: remove useEffect and do conditional setState during
  render, replace div role='button' with button element

These changes improve:
- Accessibility (semantic HTML buttons with native keyboard support)
- Code clarity (simpler conditionals, inlined single-use functions)
- React best practices (useState lazy init, no prop-to-state sync)
- Reduced LOC: 204 lines → 114 lines (-90)

One 'Back' button was using onKeyDown for both Enter and Space keys,
while all other clickable divs correctly use:
- onKeyDown for Enter (fires and repeats)
- onKeyUp for Space (fires on release, no repeat)

This inconsistency is now fixed for accessibility.