Add live stats page with metrics sampling and route split

- New sandbox_metrics_snapshots table sampled every 10s (60-day retention)
- Background MetricsSampler goroutine wired into control plane startup
- GET /v1/sandboxes/stats?range=5m|1h|6h|24h|30d endpoint with adaptive
  polling intervals; reserved CPU/RAM uses ceil(paused/2) formula
- StatsPanel component: 4 stat cards + 2 Chart.js line charts (straight
  lines, integer y-axis for running count, dual-axis for CPU/RAM)
- Range filter persisted in URL query param; polls update data silently
  (no blink — loading state only shown on initial mount)
- Split /dashboard/capsules into /list and /stats sub-routes with shared
  layout; capsuleRunningCount store syncs badge across routes
- CreateCapsuleDialog extracted as reusable component

frontend/pnpm-lock.yaml

@@ -7,6 +7,10 @@ settings:
 importers:
 
   .:
+    dependencies:
+      chart.js:
+        specifier: ^4.5.1
+        version: 4.5.1
     devDependencies:
       '@fontsource-variable/jetbrains-mono':
         specifier: ^5.2.8
@@ -249,6 +253,9 @@ packages:
   '@jridgewell/trace-mapping@0.3.31':
     resolution: {integrity: sha512-zzNR+SdQSDJzc8joaeP8QQoCQr8NuYx2dIIytl1QeBEZHJ9uW6hebsrYgbz8hJwUQao3TWCMtmfV8Nu1twOLAw==}
 
+  '@kurkle/color@0.3.4':
+    resolution: {integrity: sha512-M5UknZPHRu3DEDWoipU6sE8PdkZ6Z/S+v4dD+Ke8IaNlpdSQah50lz1KtcFBa2vsdOnwbbnxJwVM4wty6udA5w==}
+
   '@polka/url@1.0.0-next.29':
     resolution: {integrity: sha512-wwQAWhWSuHaag8c4q/KN/vCoeOJYshAIvMQwD4GpSb3OiZklFfvAgmj0VCBBImRpuF/aFgIRzllXlVX93Jevww==}
 
@@ -547,6 +554,10 @@ packages:
       '@internationalized/date': ^3.8.1
       svelte: ^5.33.0
 
+  chart.js@4.5.1:
+    resolution: {integrity: sha512-GIjfiT9dbmHRiYi6Nl2yFCq7kkwdkp1W/lp2J99rX0yo9tgJGn3lKQATztIjb5tVtevcBtIdICNWqlq5+E8/Pw==}
+    engines: {pnpm: '>=8'}
+
   chokidar@4.0.3:
     resolution: {integrity: sha512-Qgzu8kfBvo+cA4962jnP1KkS6Dop5NS6g7R5LFYJr4b8Ub94PPQXUksCw9PvXoeXPRRddRNC5C1JQUR2SMGtnA==}
     engines: {node: '>= 14.16.0'}
@@ -980,6 +991,8 @@ snapshots:
       '@jridgewell/resolve-uri': 3.1.2
       '@jridgewell/sourcemap-codec': 1.5.5
 
+  '@kurkle/color@0.3.4': {}
+
   '@polka/url@1.0.0-next.29': {}
 
   '@rollup/rollup-android-arm-eabi@4.59.0':
@@ -1203,6 +1216,10 @@ snapshots:
     transitivePeerDependencies:
       - '@sveltejs/kit'
 
+  chart.js@4.5.1:
+    dependencies:
+      '@kurkle/color': 0.3.4
+
   chokidar@4.0.3:
     dependencies:
       readdirp: 4.1.2