fix(data): route data_dir() through a per-platform shim so Android persists

dirs::data_dir() returns None on Android, which silently disabled
every persistence path (settings, stats, achievements, replays,
game-state, time-attack sessions, user themes). New
solitaire_data::platform::data_dir() shim falls through to
dirs::data_dir() on desktop and returns the per-app sandbox at
/data/data/com.solitairequest.app/files on Android — no JNI needed,
since the package id is pinned in
[package.metadata.android].

CLAUDE.md §10 already flagged this as a known pitfall; the shim
pays it down at the one chokepoint instead of per feature.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_data/src/settings.rs

@@ -400,9 +400,9 @@ impl Settings {
 }
 
 /// Returns the platform-specific path to `settings.json`, or `None` if
-/// `dirs::data_dir()` is unavailable.
+/// the platform's data directory is unavailable.
 pub fn settings_file_path() -> Option<PathBuf> {
-    dirs::data_dir().map(|d| d.join(APP_DIR_NAME).join(SETTINGS_FILE_NAME))
+    crate::data_dir().map(|d| d.join(APP_DIR_NAME).join(SETTINGS_FILE_NAME))
 }
 
 /// Load settings from an explicit path. Returns `Settings::default()` if the