feat(ui): add dockable properties panel

Preserve panel width across left/right docking and persist resize, position, and auto-collapse preferences.

Refs #348
Refs #650

assets/icons/ui/pin.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round"><path d="M8 3 H16 L15 9 L18 12 V14 H6 V12 L9 9 Z"/><path d="M12 14 V21"/></svg>