feat(engine): add InputPlugin with keyboard and stock-click

Keyboard: U=undo, N=new game, D=draw, Escape=pause placeholder (logged
only until the pause screen lands). Mouse: left-click on the stock pile
fires DrawRequestEvent. Cursor coordinates are converted via the active
Camera2d's viewport_to_world_2d so the hit-test works under arbitrary
camera setups.

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

solitaire_app/src/main.rs

@@ -1,5 +1,5 @@
 use bevy::prelude::*;
-use solitaire_engine::{CardPlugin, GamePlugin, TablePlugin};
+use solitaire_engine::{CardPlugin, GamePlugin, InputPlugin, TablePlugin};
 
 fn main() {
     App::new()
@@ -16,5 +16,6 @@ fn main() {
         .add_plugins(GamePlugin)
         .add_plugins(TablePlugin)
         .add_plugins(CardPlugin)
+        .add_plugins(InputPlugin)
         .run();
 }