feat(FN-1644): rename default global data directory from ~/.pi/fusion to ~/.fusion

- Change default global directory from ~/.pi/fusion to ~/.fusion
- Add migration logic to copy existing data from old directory to new location
- Update all core packages (store, settings, central-core, central-db) to use new default path
- Update all documentation references from ~/.pi/fusion to ~/.fusion
- Add test for ~/.pi/fusion migration path with updated mock paths
- Include changeset for @gsxdsm/fusion minor version bump

packages/dashboard/app/hooks/useTheme.ts

@@ -182,7 +182,7 @@ function unloadThemeDataStylesheet(): void {
 /**
  * Custom hook for theme management.
  *
- * Source of truth: backend global settings (`~/.pi/fusion/settings.json`).
+ * Source of truth: backend global settings (`~/.fusion/settings.json`).
  *
  * Behavior:
  * - Initializes from localStorage cache to avoid pre-hydration theme flash