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

panel/server/tests/api.rs

@@ -96,7 +96,7 @@ async fn health_and_default_manifest() {
     assert_eq!(v, "ok");
     let (s, v) = t.call("GET", "/api/v1/launcher/manifest", None, None).await;
     assert_eq!(s, StatusCode::OK);
-    assert_eq!(v["branding"]["name"], "ScopeNet");
+    assert_eq!(v["branding"]["name"], "SCOPENET");
     assert_eq!(v["api_version"], 1);
     assert!(v["user"].is_null());
 }