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/OnboardingWrapper.vue

@@ -1,5 +1,5 @@
 <template>
-  <div class="min-h-screen relative overflow-hidden">
+  <div class="onb-viewport relative overflow-hidden">
     <!-- Background layers with 3D perspective and zoom effect -->
     <div class="bg-perspective-container">
       <!-- Video background for intro/login routes (smooth transition from splash) -->
@@ -356,11 +356,20 @@ onMounted(() => {
 </script>
 
 <style scoped>
+/* Fixed viewport container — locks to screen on mobile, no bounce/overflow */
+.onb-viewport {
+  height: 100vh;
+  height: 100dvh;
+  width: 100%;
+  position: fixed;
+  inset: 0;
+}
+
 /* Wrapper to contain perspective without clipping */
 .perspective-container-wrapper {
   position: relative;
   overflow: hidden;
-  min-height: 100vh;
+  height: 100%;
 }
 
 /* Perspective container for 3D depth effect */
@@ -368,7 +377,7 @@ onMounted(() => {
   perspective: 1200px;
   perspective-origin: 50% 50%;
   position: relative;
-  min-height: 100vh;
+  height: 100%;
   overflow: hidden;
 }
 
@@ -379,10 +388,14 @@ onMounted(() => {
   transform-style: preserve-3d;
   backface-visibility: hidden;
   will-change: transform, opacity;
+  overflow: hidden;
 }
 
 .view-container {
   height: 100%;
+  overflow-y: auto;
+  overflow-x: hidden;
+  -webkit-overflow-scrolling: touch;
 }
 
 /* 2advanced-style: fluid depth transitions */