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/CloudFolder.vue

@@ -217,7 +217,6 @@ interface ContentSection {
   initialPath: string
 }
 
-const host = computed(() => window.location.hostname)
 const origin = computed(() => window.location.origin)
 
 const sections: Record<string, () => ContentSection> = {
@@ -285,12 +284,12 @@ const useNativeUI = computed(() => section.value?.nativeUI === true && appRunnin
 const iframeUrl = computed(() => section.value?.iframeUrl || '')
 
 // Initialize native file browser when entering a native-UI section
-watch(useNativeUI, async (native) => {
-  if (native && section.value) {
+watch([useNativeUI, section], async ([native, sec]) => {
+  if (native && sec) {
     cloudStore.reset()
     const ok = await cloudStore.init()
     if (ok) {
-      await cloudStore.navigate(section.value.initialPath)
+      await cloudStore.navigate(sec.initialPath)
     }
   }
 }, { immediate: true })