Restyle to a clean, flat look and rename brand to SCOPENET

- Brand is now SCOPENET everywhere (defaults, UI, installer, docs)
- Launcher: solid backgrounds (no animated orbs/grain), flat Play button
  without gradient/shine/glow, quieter rail with a simple active
  indicator, smaller type scale, Inter as default font, glass off by
  default, restyled sliders
- Panel: same flat treatment; live preview mirrors the new launcher
- New flat scope-reticle logo and app icons
- Calmer default palette
- Fix: lock the shared cargo cache in the Docker build (amd64/arm64 raced)
- Fix: Windows-only clippy lint in sys.rs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ARcGWxLx21FwXJ3yfGriS

crates/core/src/http.rs

@@ -13,7 +13,7 @@ use std::sync::Arc;
 use std::time::{Duration, Instant};
 use tokio::io::AsyncWriteExt;
 
-pub const USER_AGENT: &str = concat!("ScopeNet-Launcher/", env!("CARGO_PKG_VERSION"), " (+https://github.com/scopeddlol/SCOPENET-MC)");
+pub const USER_AGENT: &str = concat!("SCOPENET-Launcher/", env!("CARGO_PKG_VERSION"), " (+https://github.com/scopeddlol/SCOPENET-MC)");
 
 pub fn client() -> reqwest::Client {
     reqwest::Client::builder()