Presentation flow state management

### Description
Create shared state for the presentation flow, modelled after the existing issuance state in `src/state/`.

### State shape

```ts
type PresentationFlowState = {
authorizationRequest: ParsedAuthorizationRequest | null;
selectedCredentialId: string | null;
status: 'idle' | 'pending_selection' | 'pending_consent' | 'submitting' | 'success' | 'error';
error: string | null;
};
```

### Acceptance criteria
- [ ] A `PresentationFlowProvider` wraps the presentation routes
- [ ] `usePresentationFlow()` hook exposes state and actions
- [ ] State resets to `idle` on Decline or on "Go back home"
- [ ] State transitions are covered by unit tests