Fix the workspace build off-device

Two breaks that only appeared on a full `cargo test --workspace`.

`slint::android` exists only when compiling for Android, so darkroom-android
failed to compile on the host even though it is a workspace member. The entry
point is now gated on the target rather than on a feature.

The timeline forwarded `scrub` where the Timeline component declares
`scrub-to`, which the Slint compiler rejects.

Assisted-by: LLM

apps/darkroom-android/src/lib.rs

@@ -9,6 +9,11 @@
 //! * Logging goes to logcat. `env_logger` writes to stderr, which Android
 //!   discards.
 
+// `slint::android` exists only when compiling for Android, so the whole entry
+// point is gated on the target rather than on a feature. Without this the
+// crate is still a workspace member on the host, and `cargo test --workspace`
+// fails to compile it — a build break that only ever appears off-device.
+#[cfg(target_os = "android")]
 /// TRACES: M-13 | M-14
 /// Android application entry point, called by android-activity's glue.
 #[no_mangle]