fix: use XDG data directory as default storage location

Changes default from ~/.keep to /keep
(e.g. ~/.local/share/keep on Linux). Uses dirs::data_dir() which
respects XDG_DATA_HOME environment variable.

src/config.rs

@@ -573,8 +573,8 @@ impl Settings {
 
     pub fn default_dir() -> anyhow::Result<PathBuf> {
         let mut path =
-            dirs::home_dir().ok_or_else(|| anyhow::anyhow!("No home directory found"))?;
-        path.push(".keep");
+            dirs::data_dir().ok_or_else(|| anyhow::anyhow!("No data directory found"))?;
+        path.push("keep");
         if !path.exists() {
             std::fs::create_dir_all(&path)?;
         }