v0.1.0-W (vc=10): U-4 + U-5 — channels via rustypipe + rip NewPipeExtractor

channel_info(url) UniFFI suspend fn. ChannelViewModel +
SubscriptionFeedViewModel both swap. NewPipeExtractor (Java) is OUT —
zero org.schabi.newpipe classes in the APK now.

Cleanup:
- NewPipeDownloader.kt deleted (was the OkHttp adapter)
- Thumbnails.kt deleted (rustypipe returns full URLs)
- NewPipe.init() dropped from StrawApp.onCreate
- libs.newpipe.extractor removed from build.gradle.kts
- STRAW_USER_AGENT + strawHttpClient() now live in net/Http.kt
- RydClient + SponsorBlockClient + PlayerScreen + PlaybackService all
  read from net/Http.kt instead of the extractor package

rustypipe API quirks beat:
- channel_videos(id) is the right method (channel() doesn't exist)
- ChannelInfo struct = basic metadata; Channel<T> wrapper carries
  name/avatar/banner + .content is the paginator of videos
- description is String (not Option), subscriber_count is Option<u64>

End state: strawApp Kotlin is ~UI + thin glue to strawcore. The Rust
core handles search / streamInfo / channel / channel_videos via UniFFI
suspend fns. Tokio + reqwest + rustls + rquickjs all packed in
libstrawcore.so (~6MB per ABI). APK 40MB total.

rust/strawcore/src/lib.rs

@@ -8,6 +8,7 @@
 
 use std::sync::Once;
 
+mod channel;
 mod error;
 mod runtime;
 mod search;
@@ -17,6 +18,7 @@ mod stream;
 use runtime::block_on;
 
 // 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 stream::{AudioStreamItem, StreamInfo, VideoStreamItem};