feat(android): JNI clipboard bridge for Stats share-link button

Replaces the informational "Share link: {url}" toast on Android with a
real clipboard write via ClipboardManager JNI. Falls back to the old
toast on JNI error so the user can still copy the URL manually.

Adds `jni = "0.21"` (default-features = false) as a workspace dep;
`jni 0.21.1` was already in Cargo.lock as a transitive dep so no new
packages are fetched.

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

solitaire_engine/src/stats_plugin.rs

@@ -361,9 +361,13 @@ fn handle_copy_share_link_button(
     }
     #[cfg(target_os = "android")]
     {
-        toast.write(InfoToastEvent(format!(
-            "Share link: {url}"
-        )));
+        match crate::android_clipboard::set_text(&url) {
+            Ok(()) => toast.write(InfoToastEvent(format!("Copied: {url}"))),
+            Err(e) => {
+                warn!("android clipboard failed: {e}");
+                toast.write(InfoToastEvent(format!("Share link: {url}")));
+            }
+        }
     }
 }