Add storage module for presets, config, session, and history persistence

Preset save/load/list/delete/import/export, config JSON persistence,
session state save/restore, and processing history log with append/clear.
All stored as JSON under ~/.config/pixstrip/.

pixstrip-core/tests/config_tests.rs

@@ -22,6 +22,6 @@ fn config_serialization_roundtrip() {
 
 #[test]
 fn skill_level_toggle() {
-    assert_eq!(SkillLevel::Simple.is_advanced(), false);
-    assert_eq!(SkillLevel::Detailed.is_advanced(), true);
+    assert!(!SkillLevel::Simple.is_advanced());
+    assert!(SkillLevel::Detailed.is_advanced());
 }