feat: Allow signature source and target selection from downloaded and installed apps #715

Feature Description

Add options to select APKs directly from apps that are already available in the manager, instead of requiring the user to manually select APK files from internal storage.

For Signature Source, allow selecting from:

• APKs downloaded through the manager.
• APKs from installed apps, where applicable.

For Target APK, allow selecting from:

• APKs downloaded through the manager.
• Currently installed apps.

The existing option to select an APK from storage should remain available as well.

Motivation

Currently, the Signature Metadata Injector & Cloner only allows the signature source and target to be selected from APK files through the phone's file picker.

This means that if an APK has already been downloaded through the manager, it cannot be directly selected as the signature source or target. The user has to manually locate or download the APK again and then select it from internal storage.

Similarly, an installed app cannot currently be selected directly as the target.

For example, if I have already downloaded an APK through the manager, I should be able to open the signature tool and select that APK directly from the manager's downloaded apps instead of downloading/copying it again.

Ideally, the selection UI could provide something like:

Signature Source

• Downloaded Apps
• Installed Apps
• Select from Storage

Target APK

• Downloaded Apps
• Installed Apps
• Select from Storage

This would make the signature tools much more integrated with the rest of the manager and eliminate unnecessary duplicate APK downloads and manual file management.