straw: infinite-scroll pagination for channel + search

Wire the new strawcore continuation fetchers through UniFFI and add
load-more-on-scroll to the Channel and Search screens — previously both
loaded only page 1 and stopped.

FFI (rust/strawcore): search() now returns Page{items, continuation};
channelInfo carries videos_continuation; new searchContinuation() and
channelVideosContinuation() suspend funs map the core ContinuationPage.

Channel + Search ViewModels: loadMore() fetches the next page, dedups by
url, advances the token, and stops when the token runs out or a page
yields zero net-new items (guards a looping continuation). Result-set
swaps (channel switch / new submit / cache preview) cancel the in-flight
page, and a token fence inside the state update prevents a stale page
being spliced into a replaced list. Screens add a near-end LazyColumn
trigger (rememberLazyListState + derivedStateOf) and a footer spinner.

rust/strawcore/src/lib.rs

@@ -20,7 +20,7 @@ mod stream;
 // Re-exports so UniFFI sees the types at the crate root for macro discovery.
 pub use channel::ChannelInfo;
 pub use error::StrawcoreError;
-pub use search::SearchItem;
+pub use search::{Page, SearchItem};
 pub use stream::{AudioStreamItem, StreamInfo, VideoStreamItem};
 
 /// Initialize Android logging + the strawcore-core HTTP downloader.