fix(android): use reactApplicationContext.currentActivity (#69)

`ReactContextBaseJavaModule.getCurrentActivity()` was made `protected`
and `@Deprecated` in React Native 0.80+ (the recommendation in the
deprecation message is to use `reactApplicationContext.currentActivity`
directly).

In RN 0.80 with Kotlin compiling against the updated Java stub, the
Kotlin-inherited property syntax `currentActivity` no longer resolves
at the subclass level — the compiler emits `Cannot access 'getCurrentActivity':
it is protected/protected and package/package in 'ReactContextBaseJavaModule'`
or, depending on the toolchain version, a runtime crash on the
`auth.login(activity!!, ...)` call site.

Switching to `reactApplicationContext.currentActivity` (the property
the deprecation message recommends) restores the behaviour on RN 0.80+
and leaves older RN versions functionally identical, since
`reactApplicationContext` was already used elsewhere in the file
(see `getName()` / `auth` getter).

Affected entry points: `login`, `directLoginAction`, `loginWithPasskeys`,
`registerPasskeys` (4 sites).

Author: airowe