feat: rebuild the desktop interface in clipbin's style

Replaces the placeholder shell with a sidebar app in the web Clipbin's look:
a session library, a session timeline (footage, missing and cleaned-up spans,
bookmarks, a draggable clip selection exported on keyframes), clips and
settings. Geist Mono and Lucide icons are embedded. The core gains the
library model and formatting; sample data is opt-in for interface work, and
scripts/rig/screenshot-ui.sh captures each screen headlessly.

crates/clipbin-core/src/lib.rs

@@ -1,7 +1,9 @@
 //! Recorder settings and state shared with the desktop interface.
 
+pub mod library;
 mod settings;
 mod status;
 
+pub use library::Library;
 pub use settings::{Settings, VideoCodec};
 pub use status::RecordingStatus;