Mobile-responsive UI: bottom tab nav, stacked dashboard, page fixes for phones

- Layout: sidebar hidden below md; mobile top header + bottom tab bar
  (Dashboard/Activities/Health/Routes) with a More slide-up sheet holding
  Records/Import/Profile/Users, Garmin sync progress and sign-out
- Dashboard: single-column widget stack on phones ordered from the saved
  desktop layout (stat cards pair into 2-col grid); drag-edit stays
  desktop-only; grid conditionally mounted so WidthProvider never
  measures a hidden container
- New useMediaQuery/useIsMobile hook (matchMedia, md breakpoint)
- ActivityDetail: 2-col stats on phones, wrapping map toolbar, Height
  control hidden on small screens
- Activities: compact distance/time/pace line on mobile rows
- Records/Users: horizontally scrollable tables; route records hide
  Pace/Date columns below sm
- Profile forms single-column on phones; Routes expanded card stacks;
  Health flex-wrap fixes; p-4 md:p-6 page padding; h-dvh for mobile
  browser chrome
- vite.config: VITE_PROXY_TARGET override for host-side dev

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/UploadPage.jsx

@@ -142,7 +142,7 @@ function UploadZone({ title, description, accept, endpoint, icon }) {
 
 export default function UploadPage() {
   return (
-    <div className="p-6 space-y-6">
+    <div className="p-4 md:p-6 space-y-6">
       <div>
         <h1 className="text-2xl font-bold text-white">Import Data</h1>
         <p className="text-gray-500 text-sm mt-1">