With the exception of `SpotifyPlaylistActions`, moved all prop type definitions from `src/types/SpotifyPlaylistProps.ts` to the components that actually use them.

It is possible for `playlist.images == null`. This change adds null-safe handling whenever attempting to access `playlist.images[0].url`.

This refactor centralizes and generalizes async data fetching, reducing duplication and simplifying component logic.

Create custom `useFetch` hook (`src/hooks/useFetch.ts`)
- Move data-fetching logic from components into reusable hook
- Remove local useEffect-based fetch implementations from:
  - `src/components/CompareTracks.tsx`
  - `src/components/Playlists.tsx`
  - `src/components/PlaylistTracks.tsx`
- Remove component-level useState related to fetching

TODO / Follow-up work:
- Add error handling support
- Add isLoading state handling
- Introduce request cancellation via AbortController

Add AbortController and cleanup logic to useFetch
- Pass AbortSignal to fetchFn and underlying fetch calls

Update components using useFetch to receive `AbortSignal`

Update services to accept optional `signal?: AbortSignal`:
- playlistService.ts
- spotifyApi.ts
- trackService.ts

- Refactor `useFetch` to return `data = null` unless a request completes successfully

- Introduce `ErrorMessage` component for rendering Spotify API errors

- Update `useFetch` consumers to explicitly handle loading, error, and data states

- Render `<ErrorMessage />` when error is present

- Render data components only when data is non-null

Add src/hooks/useFetchSpotify.ts

Add useFetchPlaylists and useFetchTracks custom hooks

Composes Spotify service calls with generic useFetch

Removes fetch wiring logic from UI components

- Within `useFetch`, add `isLoading` to `fetchState`.
- Add Loader component
- Use Loader component within components that rely on `useFetch`

Update `AuthContext`
- Provide `accessToken` (string|null), rather than full `auth` object
  - `isAuthenticated` boolean is now based off of `accessToken` presence
  - Update corresponding components (`Playlists`, `PlaylistTracks`, `CompareTracks`) to now read `accessToken`
  - Update `useFetchPlaylists` and `useFetchTracks` to handle null `accessToken`
- Add a small token expiry buffer and `isExpiredToken` helper
- Add `setAuth(null)` within login error.

- Add `src/utils/spotifyImages.ts` with `getUriFromImages(images)`
- Rename `Header` prop `images: string[] → imageUris: Array<string | null>` and normalize inside `Header` by trimming and filtering falsy/empty URIs before rendering.
- Update `<Image>` call sites to only render when `getUriFromImages(...)` returns a non-null URI.
- Update `Header` call sites to pass `imageUris={[getUriFromImages(...)]}` (and rely on Header sanitization).

These changes correspond with the recommended `GET /playlists/{playlist_id}/items` call for fetching tracks.

https://developer.spotify.com/documentation/web-api/reference/get-playlists-tracks states that `GET /playlists/{playlist_id}/tracks` is deprecated.