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/Cargo.toml

@@ -19,6 +19,7 @@ webp = "0.3"
 little_exif = "0.4"
 imageproc = "0.25"
 ab_glyph = "0.2"
+dirs = "6"
 
 [dev-dependencies]
 tempfile = "3"