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/ActivitiesPage.jsx

@@ -41,7 +41,7 @@ export default function ActivitiesPage() {
   const clearDateFilter = () => navigate('/activities')
 
   return (
-    <div className="p-6">
+    <div className="p-4 md:p-6">
       <div className="flex items-center justify-between mb-4">
         <h1 className="text-2xl font-bold text-white">Activities</h1>
         <Link
@@ -54,7 +54,7 @@ export default function ActivitiesPage() {
 
       {/* YTD stats */}
       {ytdStats && (
-        <div className="flex gap-4 mb-4 text-sm">
+        <div className="flex flex-wrap gap-x-4 gap-y-1 mb-4 text-sm">
           {ytdStats.running_km > 0 && (
             <span className="text-blue-400">🏃 {ytdStats.running_km.toFixed(0)} km this year</span>
           )}
@@ -100,7 +100,7 @@ export default function ActivitiesPage() {
             <Link
               key={activity.id}
               to={`/activities/${activity.id}`}
-              className="flex items-center gap-4 bg-gray-900 hover:bg-gray-800 border border-gray-800 hover:border-gray-700 rounded-xl p-4 transition-all group"
+              className="flex items-center gap-3 p-3 sm:gap-4 sm:p-4 bg-gray-900 hover:bg-gray-800 border border-gray-800 hover:border-gray-700 rounded-xl transition-all group"
             >
               {/* Sport indicator */}
               <div
@@ -116,6 +116,10 @@ export default function ActivitiesPage() {
                   {activity.name}
                 </p>
                 <p className="text-xs text-gray-500 mt-0.5">{formatDate(activity.start_time)}</p>
+                {/* Compact metrics line — the full metrics column is hidden below sm */}
+                <p className="sm:hidden text-xs text-gray-400 mt-0.5 truncate">
+                  {formatDistance(activity.distance_m)} · {formatDuration(activity.duration_s)} · {formatPace(activity.avg_speed_ms, activity.sport_type)}
+                </p>
               </div>
 
               {/* Metrics */}