feat(app): persist window geometry across launches

Settings gains an optional window_geometry field (size + position)
serialized via #[serde(default)] so legacy settings.json files without
the field deserialize cleanly to None. On launch the app restores
the persisted dimensions and position; first run and pre-upgrade
saves keep the existing 1280x800 centered default.

settings_plugin records changes from WindowResized and WindowMoved
into a PendingWindowGeometry resource and writes them to disk through
the existing atomic .tmp+rename path once the events have stayed
quiet for WINDOW_GEOMETRY_DEBOUNCE_SECS (0.5s). A merge_geometry
helper preserves whichever component (size or position) the latest
event burst didn't carry, so a position-only WindowMoved never wipes
the recorded size.

Pure should_persist_geometry and merge_geometry helpers are unit
tested for the boundary cases. Headless integration tests cover the
full flow: a single resize event then a quiet window persists, a
move event after a resize updates only position, a rapid storm
collapses to the final size, and a quiet frame with no events
leaves the geometry untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_data/src/lib.rs

@@ -126,7 +126,7 @@ pub use challenge::{challenge_count, challenge_seed_for, CHALLENGE_SEEDS};
 pub mod settings;
 pub use settings::{
     load_settings_from, save_settings_to, settings_file_path, AnimSpeed, Settings, SyncBackend,
-    Theme,
+    Theme, WindowGeometry,
 };
 
 pub mod auth_tokens;