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/Cargo.toml

@@ -32,6 +32,9 @@ zip            = { workspace = true }
 [target.'cfg(not(target_os = "android"))'.dependencies]
 arboard        = { workspace = true }
 
+[target.'cfg(target_os = "android")'.dependencies]
+jni            = { workspace = true }
+
 [dev-dependencies]
 async-trait = { workspace = true }
 tempfile    = { workspace = true }