Portfolio persistence

The Portfolio data is crucial to the user. While it's frequently feasible for users to manually recover it, discovering that financial data has been lost due to an upgrade or other circumstances can be frustrating.

We have two options here:
* Store the data within the standard Android app folder.
* Allow users to select a preferred folder for data storage.

While both options facilitate future device synchronization, the latter also enables the use of third-party syncing mechanisms, adding an extra layer of versatility.

The issue names no files, tests, or entry points. First decide whether Portfolio data should use the standard Android app folder or a user-selected folder, then locate the existing Portfolio persistence flow and define how upgrades, recovery, and synchronization should behave.

Written by the indexing model from the issue text.