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/style.css

@@ -446,6 +446,28 @@ input[type="radio"]:active + * {
     overflow-x: hidden;
   }
 
+  /* Onboarding scroll container — constrains to viewport and scrolls */
+  .onb-scroll-container {
+    max-height: calc(100dvh - 2rem);
+    overflow-y: auto;
+    overflow-x: hidden;
+    -webkit-overflow-scrolling: touch;
+  }
+
+  /* Mobile: tighter padding and margins for onboarding containers */
+  @media (max-width: 639px) {
+    .path-glass-container {
+      width: calc(100% - 24px);
+      margin: 12px auto;
+      padding: 12px;
+      border-radius: 20px;
+      gap: 8px;
+    }
+    .onb-scroll-container {
+      max-height: calc(100dvh - 1.5rem);
+    }
+  }
+
   /* Choose Your Path - Option Cards */
   .path-option-card {
     position: relative;
@@ -585,6 +607,21 @@ input[type="radio"]:active + * {
     font-weight: 600;
   }
 
+  @media (max-width: 639px) {
+    .path-action-button {
+      font-size: 15px;
+      border-radius: 14px;
+    }
+    .path-action-button--skip {
+      padding: 10px 24px;
+      font-size: 14px;
+    }
+    .path-action-button--continue {
+      padding: 12px 28px;
+      font-size: 15px;
+    }
+  }
+
   .path-action-button::before {
     content: '';
     position: absolute;