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/journal/JournalPanel.tsx

@@ -1,7 +1,8 @@
 /**
  * JournalPanel — right panel container for the journal stream
  *
- * Fixed overlay. Shows stream when connected, connect dialog when not.
+ * On mobile: fixed overlay with backdrop.
+ * On desktop: positioned by the parent flex container (width transition).
  * Auto-connects to ws://{current host}:{current port} — no URL input needed.
  */
 
@@ -33,19 +34,19 @@ export const JournalPanel: Component<JournalPanelProps> = (props) => {
 
   return (
     <>
-      {/* Backdrop — only on mobile, starts below header */}
+      {/* Mobile backdrop — overlay that appears below the header */}
       <div
         class="fixed top-16 inset-x-0 bottom-0 bg-black/30 z-40 md:hidden"
         classList={{ hidden: !props.open }}
         onClick={props.onClose}
       />
-      {/* Panel — always mounted for exit animation, visibility toggled */}
+      {/* Panel — on mobile: fixed overlay; on desktop: fills the parent wrapper */}
       <aside
-        class={`fixed top-16 right-0 bottom-0 z-50 bg-white border-l border-gray-200
-               flex flex-col w-full max-w-md md:w-105 shadow-lg
-               transition-transform duration-300 ease-in-out ${
-                 props.open ? "translate-x-0" : "translate-x-full"
-               }`}
+        class={`fixed top-16 right-0 bottom-0 z-50 bg-white
+               flex flex-col w-full h-full max-w-md shadow-lg
+               transition-transform duration-300 ease-in-out
+               md:static md:inset-auto md:w-105 md:max-w-none md:shadow-none md:border-l md:border-gray-200
+               ${props.open ? "translate-x-0" : "translate-x-full"}`}
         inert={!props.open}
       >
         <JournalContext.Provider value={{ onClose: props.onClose }}>