v1.3.5: own API key on the phone no longer hangs (secure-storage plugin handed to a promise as a thenable)

Every key-store call on Android stalled before it started: the Capacitor plugin proxy answers
`then` with a native method, so a promise resolving to the plugin itself never settled. Save
greyed out, key "not saved", Coach "thinking" forever (#42, #58). The plugin now travels inside
a plain object; a test drives the store through a Capacitor-style proxy. Verified on an emulator.