Resolve every base directory in one place, and on Windows

Five sites each read XDG_*_HOME and fell back to $HOME/.local/… on
their own, which is fine on Linux and wrong everywhere else: Windows
sets neither variable, so every one of them degraded to a path
relative to the working directory — for a Start Menu launch,
C:\Windows\System32. The models lookup walked XDG_DATA_DIRS the same
way.

dr_plat::dirs now holds the rule per platform: XDG on Unix, the known
folders on Windows — %APPDATA% for config, which roams, and
%LOCALAPPDATA% for data and state, which do not — and the executable's
own directory as the system data dir, which is where the installer
puts the models. The Android overrides stay where they were; only the
fallback behind them moved. Both rule sets are unit-tested on either
host, and the Windows one was confirmed by running the application
under Wine: its log landed in AppData\Local\darkroom\state and nothing
was written anywhere else.

ui/dr-ui/src/settings_store.rs

@@ -25,17 +25,12 @@ pub struct SettingsStore {
 impl SettingsStore {
     /// Open the store at the platform config location.
     ///
-    /// Linux: `$XDG_CONFIG_HOME/darkroom/settings.json`, falling back to
-    /// `~/.config` (FR-PLAT-LIN-1) — the same resolution `SessionStore` does,
-    /// so the two files sit together and a user backing up one takes both.
+    /// `settings.json` in the platform's config directory — `dr_plat::dirs`
+    /// has the rule per platform — which is the same resolution `SessionStore`
+    /// makes, so the two files sit together and a user backing up one takes
+    /// both.
     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("settings.json"))
+        Self::open_at(dr_plat::base_dir(dr_plat::Base::Config).join("settings.json"))
     }
 
     /// Open at an explicit path — for tests, and for a non-default location.