Take the app half from wg-app-link as well

The four Kotlin files that were the link rather than this product now come
from the submodule: the pinned TrustManager, the enrollment store and its
Keystore sealing, the QR capture activity, and the local-network permission
check. `:link` is a subproject resolved by path, so the app half is
version-locked to the same commit the Rust half already was.

What stays here is the two facts that are actually about this app, and
both are load-bearing in a way that would fail quietly if got wrong: the
`aiapp` URI scheme, and the Keystore alias `aiapp-token-key` that every
enrolled phone's token is already sealed under. A wrong alias would leave
those phones reading as not enrolled with nothing on screen to explain it,
so the value is carried over exactly and the reason is written beside it.

Call sites are unchanged. `ServerSettings` stays available unqualified as
a typealias and `applyPinnedTls()` stays an extension, so the diff is the
three files that bind the product-specific values plus two imports --
rather than every screen that happens to use a setting.

Also clears a warning the build had been printing: `setup?.id.orEmpty()`
where the compiler already knows `setup` is non-null, because `chosen`
came from that setup's own provider list.

Verified by running the build, not only by reading it: ktfmt, Kotlin
compile and Android Lint are all clean with no warnings, and the APK still
builds -- which exercises the pinned-CA generator, since that is the step
that reads the CA off this machine.

Still unpushed, per the hold until the rebuild bug is proven fixed. Note
the submodule: a checkout of this commit needs `git submodule update
--init` before `app/` or `server/` will build.

app/androidApp/src/main/kotlin/com/example/aiapp/SpawnScreen.kt

@@ -286,7 +286,11 @@ fun SpawnScreen(
                                     // The id, not the label: labels are
                                     // editable and the server resolves by
                                     // id.
-                                    setup = setup?.id.orEmpty(),
+                                    // Non-null here: `chosen` came from
+                                    // `setup`'s own provider list, so
+                                    // reaching this point proves there was
+                                    // a setup to take it from.
+                                    setup = setup.id,
                                     provider = chosen.name,
                                     title = title.trim(),
                                     model =