Save presets beside the settings, not under a raw HOME

PresetStore::open built its path from XDG_CONFIG_HOME or HOME. Android
sets neither, so the library resolved to /.config/darkroom, which is
read-only, and every preset saved on the tablet failed. Windows sets no
HOME either and got a directory relative to the working directory. The
settings store was moved to dr_sync::account::config_dir for the same
reason in 0.12.1; the presets now follow it. Linux and macOS resolve to
the same file as before.

ui/dr-ui/src/preset_store.rs

@@ -27,17 +27,14 @@ pub struct PresetStore {
 impl PresetStore {
     /// Open the store at the platform config location.
     ///
-    /// Linux: `$XDG_CONFIG_HOME/darkroom/presets.drpl`, falling back to
-    /// `~/.config` — the same resolution `SettingsStore` does, so the files sit
-    /// together and a user backing up one takes all of them.
+    /// Beside `settings.json`, in [`dr_sync::account::config_dir`], so a user
+    /// backing up one takes all of them. Never from `HOME` directly: Android
+    /// declares its data directory rather than setting `HOME`, and a path
+    /// built from an empty one is `/.config`, which is read-only — every
+    /// preset saved on the tablet failed. Windows sets no `HOME` either, and
+    /// got a directory relative to wherever the app was started.
     pub fn open() -> Self {
-        let dir = std::env::var_os("XDG_CONFIG_HOME")
-            .map(PathBuf::from)
-            .unwrap_or_else(|| {
-                PathBuf::from(std::env::var("HOME").unwrap_or_default()).join(".config")
-            })
-            .join("darkroom");
-        Self::open_at(dir.join(format!(
+        Self::open_at(dr_sync::account::config_dir().join(format!(
             "presets.{}",
             dr_pipeline::preset::LIBRARY_EXTENSION
         )))