fix(protocol,storage): platform-aware client version and Android stub (TODO-010,011)

pick_client_version() now returns correct version per platform via
cfg(target_os). keyring_load() on unsupported platforms returns
StorageError instead of silently returning Ok(None).

crates/chanora_storage/src/lib.rs

@@ -270,7 +270,9 @@ impl IdentityFileStore {
         target_os = "ios"
     )))]
     fn keyring_load(&self) -> Result<Option<[u8; 32]>, StorageError> {
-        Ok(None)
+        Err(StorageError::SecureStore(
+            "keyring is not yet supported on this platform".into(),
+        ))
     }
 
     /// Persist the DEK in the platform keyring. Returns true on