Add delete all user fingerprints option

[google-labs-jules]

This change adds a new "Delete All User's Prints" option to the application.
It includes:

1. A new Page variant DeleteAllUsersPrints which appears in the navigation bar.
2. A delete_fingers function in src/app/fprint.rs that iterates through all enrolled fingerprints for a user and deletes them.
3. Logic in src/app/mod.rs to handle this new page:
   • The "Delete" button is enabled on this page if the user has any enrolled fingerprints.
   • The "Register" button is disabled on this page.
   • Clicking "Delete" on this page triggers the bulk deletion process.
   • Successful deletion clears the local list of enrolled fingerprints.
4. Localization for the new page label.
5. Refactoring Page::as_finger_id() to return Option<&'static str> to accommodate non-finger pages.
6. Updated unit tests to verify the changes.

Fixes #72

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PR created automatically by Jules for task 2767584120024299952 started by @jotuel

[google-labs-jules]

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