feat: scope Build file browser to selected app, rename Apps → Build

- Sidebar "Apps" section renamed to "Build"
- Each app now links to /build?app=<name>&root=<path> so the browser
  opens scoped to that app's subdirectory only
- Build page shows an empty-state prompt when no app is selected
- File tree header shows the selected app name, breadcrumb shows
  relative path within the app (strips the root prefix)
- Wraps useSearchParams in Suspense for Next.js static rendering

Made-with: Cursor

components/layout/vibn-sidebar.tsx

@@ -312,15 +312,15 @@ export function VIBNSidebar({ workspace }: VIBNSidebarProps) {
               </div>
             )}
 
-            {/* ── Apps ── */}
-            <SectionHeading label="Apps" collapsed={collapsed} />
+            {/* ── Build ── */}
+            <SectionHeading label="Build" collapsed={collapsed} />
             {apps.length > 0 ? (
               apps.map(app => (
                 <SectionRow
                   key={app.name}
                   icon="▢"
                   label={app.name}
-                  href={`${base}/build`}
+                  href={`${base}/build?app=${encodeURIComponent(app.name)}&root=${encodeURIComponent(app.path)}`}
                   collapsed={collapsed}
                 />
               ))