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>

Cargo.toml

@@ -31,6 +31,7 @@ keyring     = "4"
 keyring-core = "1"
 reqwest     = { version = "0.13", features = ["json", "rustls", "rustls-native-certs"], default-features = false }
 arboard     = { version = "3", default-features = false }
+jni         = { version = "0.21", default-features = false }
 
 solitaire_core   = { path = "solitaire_core" }
 solitaire_sync   = { path = "solitaire_sync" }