fix: check credential storage availability before access preferences

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

Update FontHelper and related methods to access storage safely.

Tests performed

• Tested Fossify Keyboard on lock screen after reboot

Closes the following issue(s)

• Closes Keyboard crashes when attempting to decrypt the device after rebooting Keyboard#59

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.