fix: mobile onboarding viewport + filebrowser demo fixes

Onboarding:
- Fixed viewport to use dvh units with position:fixed container
- All views use scrollable glass containers that fit within viewport
- Responsive typography and spacing (mobile-first breakpoints)
- Tighter padding/margins on small screens
- RootRedirect checks localStorage first for instant redirect
- Spinner only appears after 500ms delay to avoid flash

Filebrowser:
- Fix CloudFolder null initialPath crash (watch both useNativeUI + section)
- Remove unused `host` computed (was causing TS error)
- Add mock GET /app/filebrowser/ landing page
- Increase express.json limit to 50mb

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

neode-ui/src/views/OnboardingPath.vue

@@ -1,7 +1,7 @@
 <template>
-  <div class="h-screen flex items-center justify-center p-3 sm:p-4 md:p-6 relative overflow-hidden">
+  <div class="min-h-full flex items-center justify-center p-3 sm:p-4 md:p-6 relative">
     <!-- Main Glass Container - Scrollable -->
-    <div class="max-w-[1200px] w-full relative z-10 path-glass-container max-h-[calc(100vh-2rem)] sm:max-h-[calc(100vh-3rem)] overflow-y-auto overflow-x-hidden">
+    <div class="max-w-[1200px] w-full relative z-10 path-glass-container onb-scroll-container">
       <!-- Header -->
       <div class="text-center mb-4 md:mb-6 flex-shrink-0 px-3 sm:px-4 pt-4 sm:pt-6">
         <h1 class="text-xl md:text-[26px] font-semibold text-white/96 mb-2 drop-shadow-[0_2px_6px_rgba(0,0,0,0.4)]">Your Node, Your Possibilities</h1>