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.

core/dr-sync/src/account.rs

@@ -67,15 +67,21 @@ pub fn set_data_dir(dir: PathBuf) {
     let _ = DATA_DIR.set(dir);
 }
 
+/// The directory the platform entry point declared, if it declared one.
+///
+/// Android does; a desktop does not, and resolves through `dr_plat::dirs`
+/// instead. Exposed so a caller wanting the *data* directory can honour the
+/// same declaration without inheriting the config rule as its fallback.
+pub fn declared_data_dir() -> Option<PathBuf> {
+    DATA_DIR.get().cloned()
+}
+
 /// The directory configuration lives in.
 pub fn config_dir() -> PathBuf {
     if let Some(d) = DATA_DIR.get() {
         return d.clone();
     }
-    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")
+    dr_plat::base_dir(dr_plat::Base::Config)
 }
 
 /// TRACES: FR-NC-12