Introduce a new Xbox-style guide (rail + expanded panel) to the shell: adds guideSidebar component, data model, styles, and panel overlay (src/core/guideSidebar.js, src/core/sidebarData.js, src/styles/guide.css, src/views/overlays/guidePanel.*). Integrates the guide into the app lifecycle and routing (main.js): mounts the guide, remaps focus roots, adds guide open/close behavior, and registers placeholder views for store/mods. Input and navigation updated to support a dedicated "guide" action and region (src/core/input.js, src/core/nav.js) so focus movement and acceptance work inside the guide. index.html updated to mount the guide and include templates/styles; base/component CSS adjusted to accommodate the new mount and focus states. Misc: adds a placeholder view and wiring for guide panel overlays and tweaks power menu handling to cooperate with the guide.
