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.

README.md

@@ -8,7 +8,7 @@ An open-source game recorder for Windows and Linux, inspired by Plays.tv. Rust a
 
 The recording foundation is being validated (see [docs/validation.md](docs/validation.md)). `clipbin-obs` records official OBS capture sources through one hardware encoder into keyframe-aligned segments and reports honest status; `clipbin-probe` exercises it. It passes the Linux headless rig and the hardware sign-off on CachyOS/Hyprland/RX 9070 XT; all Windows validation is still to do.
 
-The desktop app is still the initial shell: it opens a GPUI window, displays the agreed recording defaults, and always reports **Not recording**. Steam detection, the recorder in the app, playback, clipping, bookmarks, persistence, global hotkeys, autostart, and background recording are not implemented yet.
+The desktop interface follows the web Clipbin's look (zinc dark theme, purple accent, Geist Mono): a session library, a session timeline with bookmarks and a clip selection, clips, and settings. It is not yet connected to the recorder, so it shows empty states; `CLIPBIN_SAMPLE_LIBRARY=1` fills it with example sessions for interface work. Steam detection, the recorder in the app, playback, export, persistence, global hotkeys, autostart, and background recording are not implemented yet.
 
 Windows is a first-class target. Every change must keep the Windows code paths building (`cargo clippy --target x86_64-pc-windows-msvc`) and list what still needs validating on Windows.
 
@@ -85,7 +85,7 @@ cargo test --locked --workspace
 cargo clippy --locked --target x86_64-pc-windows-msvc -p clipbin-core -p clipbin-obs -p clipbin-probe --all-targets -- -D warnings
 ```
 
-Recording validation on Linux, headless: `scripts/validate/linux-rig.sh`.
+Recording validation on Linux, headless: `scripts/validate/linux-rig.sh`. Interface screenshots, headless: `scripts/rig/screenshot-ui.sh` (screens: sessions, session, clips, settings, failed, empty).
 
 The core can also be checked without native UI dependencies: