feat(frontend): Transform layout to use shadcn dashboard-01 with iOS PWA safe area support.

- Replace custom layout with modern SidebarProvider/SidebarInset structure
- Add AppSidebar component using shadcn patterns with preserved account summary
- Add SiteHeader component with SidebarTrigger integration
- Install shadcn sidebar, separator, and related UI components
- Fix iOS PWA safe area issues - sidebar no longer overlaps dynamic island/notch
- Add pt-safe-top and pl-safe-left classes to sidebar and header components
- Remove legacy Sidebar.tsx and Header.tsx components
- Preserve all existing functionality: navigation, API health status, theme toggle, PWA features
- Improve mobile responsive behavior with built-in shadcn drawer patterns

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/hooks/use-mobile.tsx

@@ -0,0 +1,19 @@
+import * as React from "react"
+
+const MOBILE_BREAKPOINT = 768
+
+export function useIsMobile() {
+  const [isMobile, setIsMobile] = React.useState<boolean | undefined>(undefined)
+
+  React.useEffect(() => {
+    const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT - 1}px)`)
+    const onChange = () => {
+      setIsMobile(window.innerWidth < MOBILE_BREAKPOINT)
+    }
+    mql.addEventListener("change", onChange)
+    setIsMobile(window.innerWidth < MOBILE_BREAKPOINT)
+    return () => mql.removeEventListener("change", onChange)
+  }, [])
+
+  return !!isMobile
+}