refactor: implement scroll container context and flex layout

Switch from fixed positioning to a flexbox-based layout for the
main application structure. This introduces a `ScrollContainerCtx` to
provide access to the main scrollable element, allowing child
components like `FileTree` and `RevealManager` to perform accurate
scrolling and positioning relative to the container instead of the
window.

refactor: set JournalPanel height to full

src/components/Sidebar.tsx

@@ -158,7 +158,7 @@ export const MobileSidebar: Component<SidebarProps> = (props) => {
 };
 
 /**
- * 桌面端固定侧边栏
+ * 桌面端侧边栏 — flex child, no fixed positioning
  */
 export const DesktopSidebar: Component<{
   fileTree?: FileNode[];
@@ -182,7 +182,7 @@ export const DesktopSidebar: Component<{
   });
 
   return (
-    <aside class="hidden md:block fixed top-0 left-0 h-full w-64 bg-white shadow-lg z-30 overflow-hidden pt-16">
+    <aside class="hidden md:flex flex-col w-64 shrink-0 bg-white shadow-lg overflow-hidden">
       <SidebarContent
         fileTree={fileTree()}
         pathHeadings={pathHeadings()}