Add Nebula guide sidebar and navigation

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.

src/core/sidebarData.js

@@ -0,0 +1,58 @@
+/** Primary shell navigation (data-driven). */
+export const PRIMARY_NAV = [
+  { id: "home", label: "Home", icon: "⌂", target: "home" },
+  { id: "library", label: "Library", icon: "⊟", target: "library" },
+  { id: "store", label: "Store", icon: "⊞", target: "store" },
+  { id: "mods", label: "Mods", icon: "◇", target: "mods" },
+  { id: "settings", label: "Settings", icon: "⚙", target: "settings" },
+];
+
+/** Quick actions in the expanded guide. */
+export const QUICK_ACTIONS = [
+  { id: "search", label: "Search", icon: "⌕", panel: "search" },
+  { id: "notifications", label: "Notifications", icon: "🔔", panel: "notifications" },
+  { id: "downloads", label: "Downloads", icon: "↓", panel: "downloads" },
+  { id: "controller", label: "Controller Settings", icon: "🎮", panel: "controller" },
+  { id: "power", label: "Power Menu", icon: "⏻", action: "power" },
+];
+
+/** Mock recently played titles (replace with library bridge later). */
+export const RECENT_ITEMS = [
+  {
+    id: "recent-halo",
+    title: "Halo Infinite",
+    source: "steam",
+    lastPlayed: "2 hours ago",
+    accent: "#1e4a6e",
+  },
+  {
+    id: "recent-cyber",
+    title: "Cyberpunk 2077",
+    source: "gog",
+    lastPlayed: "Yesterday",
+    accent: "#5c1a3a",
+  },
+  {
+    id: "recent-fortnite",
+    title: "Fortnite",
+    source: "epic",
+    lastPlayed: "3 days ago",
+    accent: "#2a3a5c",
+  },
+  {
+    id: "recent-emulator",
+    title: "Nebula Arcade",
+    source: "local",
+    lastPlayed: "Last week",
+    accent: "#1a3c2e",
+  },
+];
+
+export const SOURCE_LABELS = {
+  steam: "Steam",
+  gog: "GOG",
+  epic: "Epic",
+  local: "Local",
+};
+
+export const NAVIGABLE_VIEWS = new Set(PRIMARY_NAV.map((item) => item.target));