Improve floor grid and add dev tooling

- Scale floor perspective with viewport height so grid reaches horizon
  on full-screen KDE (was broken with fixed 700px perspective value)
- Increase grid line thickness 1px → 2px to reduce sub-pixel flickering
- Remove DANCEBOT-9000 title text
- Add viewport meta tag for correct layout in WebEngineView
- Add dev-server.sh (port 3000) with in-browser music ticker shim

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dev-server.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+PORT=${1:-3000}
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/plugin/contents/ui" && pwd)"
+
+echo "==> Serving $DIR on http://localhost:$PORT/dancer.html"
+echo "    Press Ctrl+C to stop."
+echo ""
+
+cd "$DIR"
+exec python3 -m http.server "$PORT"