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/lib.rs

@@ -163,3 +163,6 @@ pub use replay::{
     replay_history_path, save_replay_history_to, Replay, ReplayHistory, ReplayMove,
     REPLAY_HISTORY_CAP, REPLAY_HISTORY_SCHEMA_VERSION, REPLAY_SCHEMA_VERSION,
 };
+
+pub mod platform;
+pub use platform::data_dir;