Amogh/Implement sharing of locations with other users

[Amoghhosamane]

#26

Pull Request Details

Description

Implement sharing of locations with other Solid Pod users as described in #26.

• Allows users to share their own locations (both plaintext JSON and encrypted TTL files) with other WebIDs or groups using solidui's GrantPermissionUi.
• Discovers locations shared with the current user by scanning Solid Pod permission logs (sharedResources()).
• Displays shared locations on the map using a distinct color (Colors.deepPurple) and shows who shared the place in the marker details sheet.
• Adds a dedicated "Shared" locations screen (ListExternalPlacesScreen) in the navigation rail/drawer for searching, viewing, and re-sharing external places.
• Adds a direct Share button to location list tiles and the location detail dialog on the Locations page.
• Handles remote content retrieval for both plaintext and encrypted external files.

Related Issues

Closes #26

Type of Change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How To Test?

1. Run flutter test to execute all unit tests, including new tests in test/sharing_test.dart.
2. Run flutter analyze lib test to verify zero lint or static analysis issues.
3. Log in with a Solid Pod account that has places shared with it:
   • Navigate to the Shared menu to view, search, and inspect places shared by other users.
   • Check the Home map: verify that shared places appear with a deep purple marker.
   • Tap a shared marker to inspect its bottom sheet: verify that the "Shared with you by: " banner appears.
4. On the Locations page, tap the Share icon on any saved place to share it with another WebID.

Checklist

• Changes adhere to the style and coding guidelines
• I have performed a self-review of my code
• Added unit tests for shared places and marker rendering
• Updated CHANGELOG.md and bumped version in pubspec.yaml