Allow saving a stored banner image in Manage Theme

[VincentBorgers]

Summary

• Saving the Manage Theme settings page fails once a banner image is stored, with The banner image field contains a file path that is not permitted. This also blocks replacing or removing the banner (Banner image upload fails with "The banner image field contains a file path that is not permitted" #467, Site banner change bug #471).
• The app_banner FileUpload uses preventFilePathTampering(). On a settings page the existing stored path is not registered as an allowed path, so the guard rejects it on every save, even when nothing changed.
• Pass allowFilePathUsing to permit a path when the file actually exists on the configured uploads disk. Tampered or non-existent paths are still rejected; a stored banner is accepted again. The closure uses the same disk the field already uploads to.
• Add a regression test that saves the page with a banner already stored.

Validation

• The new test fails on main with the error above and passes with the fix.
• vendor/bin/pest: 1145 passed (4237 assertions), after composer build.
• vendor/bin/pint --test: passed.
• vendor/bin/phpstan analyse src/Filament/Pages/Settings/ManageTheme.php: no errors.

Fixes #467
Fixes #471