Standardize preferences storage paths across all platforms (macOS, Windows, Linux)

[haiphucnguyen]

Summary

Currently, ApplicationPreferences and AccountPreferences both rely on Java's Preferences API, which writes to ~/.java/.userPrefs/io/askimo/... on all platforms. This path is non-standard on every OS and causes friction - especially on Linux where XDG compliance is expected.

Current behavior

Storage	Path
Java Preferences (ApplicationPreferences, AccountPreferences)	~/.java/.userPrefs/io/askimo/app/...
App data (AskimoHome)	~/.askimo/personal/

Expected behavior (per-platform convention)

Platform	Preferences path	Notes
macOS	~/Library/Application Support/Askimo/	Standard for macOS apps
Windows	%APPDATA%\Askimo\	Standard for Windows apps
Linux	$XDG_CONFIG_HOME/askimo/ (default ~/.config/askimo/)	XDG Base Directory spec

Scope

• ApplicationPreferences — UI layout, onboarding state
• AccountPreferences — sync cursors, model selection, plan inputs, star prompt state, hardware acceleration flags, UI scale

Proposed behavior

Class	New storage location
ApplicationPreferences	<AskimoHome.base()>/prefs/app.properties
AccountPreferences (per account)	<AskimoHome.base()>/prefs/accounts/<accountId>.properties
AccountPreferences (device)	<AskimoHome.base()>/prefs/accounts/__device__.properties

AskimoHome.base() already resolves to ~/.askimo/personal/ by default and respects the ASKIMO_HOME env override - no new path logic needed.

Benefits

• Consistent with AskimoHome - all app data under one known directory
• Fully testable via the existing AskimoHome.withTestBase() test infrastructure - no system property hacks needed in tests
• No global JVM-level side effects (unlike a custom PreferencesFactory)
• Users can easily find, back up, and inspect their preferences as plain text files
• If user remove Askimo, then they delete ~/.askimo, everything gone, no orphaned data scattered across other folders.