Android: Add folder/album browsing for photo impor

[Jaibar]

Problem

When importing photos for a dive on Android, the app auto-scans the device gallery using a 30-minute timestamp buffer around the dive time. This picks up all images on the phone within that window —
WhatsApp memes, browser downloads, screenshots, etc. — making it hard to find the actual dive photos.

The permission dialog offers "limited" or "full" access, but there is no way to browse a specific folder (e.g. DCIM/GoPro/exports) to select photos manually.

On desktop (Windows/Linux), the file picker already allows navigating to any folder, so this is an Android/iOS-only gap.

Suggested improvements

1. Add album/folder filtering — photo_manager already supports listing albums via PhotoManager.getAssetPathList(). Add a dropdown or sidebar to filter by album (Camera, GoPro, Downloads,
   etc.) instead of showing everything flat.

2. Add a "Browse files" option — alongside the gallery picker, offer a file-based picker (e.g. file_picker package) that lets users navigate to a specific directory.

3. Combine both — keep timestamp auto-match for convenience, but add album filtering and a manual file browse option.

Current behavior

• PhotoPickerServiceMobile uses photo_manager to fetch all photos within the time window
• No folder/album filtering is available in PhotoPickerPage
• Desktop already uses image_picker with folder navigation

Relevant files

• lib/features/media/data/services/photo_picker_service_mobile.dart
• lib/features/media/presentation/pages/photo_picker_page.dart
• lib/features/media/presentation/providers/photo_picker_providers.dart