fix(desktop): store preferences under Askimo home

[AnayGarodia]

What changed

• replace Java Preferences storage with atomic plain .properties files under AskimoHome.base()
• store application, per-account, and device preferences at the paths proposed in the issue
• preserve typed getters, defaults, account isolation, and reset behavior
• add tests covering paths, account ID sanitization, persistence, and device isolation

Verification

Using Temurin JDK 25, as required by the current Gradle toolchain:

• ./gradlew :desktop-shared:test
• ./gradlew :desktop-shared:spotlessCheck
• ./gradlew :desktop-shared:detekt

All tasks completed successfully.

Closes #531

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[haiphucnguyen]

Thank you @AnayGarodia !