External Sidebar Window Playground preview

Corrected preview: Sidebar Window now opens as a real sibling OpenStation window outside the source editor. The source editor snaps left, the ephemeral companion opens right, and closing either side restores lifecycle state.\n\nValidated with lint, 3,793 JavaScript tests, a production build, and package integrity checks.\n\nSHA-256: 2a5d522315db7957368d86464547edd0e59bf8113a4ad05b0bf5558a068192b7