fix(android): remove unused JValue import and fix match arm types

Two cfg(android) issues hidden from Linux CI:
- android_clipboard.rs: JValue was imported but never used (JValueOwned
  covers all call sites). Removed to satisfy -D unused-imports.
- stats_plugin.rs: both arms of the clipboard match now return () via
  explicit block+semicolon, resolving the type mismatch that pinged-pong
  between runs due to bidirectional match-arm type inference.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/android_clipboard.rs

@@ -6,7 +6,7 @@
 pub fn set_text(text: &str) -> Result<(), String> {
     use bevy::android::ANDROID_APP;
     use jni::{
-        objects::{JObject, JValue, JValueOwned},
+        objects::{JObject, JValueOwned},
         JavaVM,
     };