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/vite.config.js

@@ -6,7 +6,8 @@ export default defineConfig({
   server: {
     proxy: {
       '/api': {
-        target: 'http://backend:8000',
+        // 'backend' only resolves inside docker compose; override for host-side dev
+        target: process.env.VITE_PROXY_TARGET || 'http://backend:8000',
         changeOrigin: true,
       },
     },