fix: keep all tabs mounted, switch via CSS display instead of unmount

All 4 tabs (Dashboard, Studio, Shell, Config) are now always mounted
and toggled via .tab-hidden (display:none). This preserves:
- Dashboard graph history across tab switches
- Terminal session state and progress
- Studio chat context
- Config form state

Dashboard polling pauses after 3 ticks when hidden to save resources
and auto-resumes when the tab becomes visible again.

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

web/src/components/Dashboard.jsx

@@ -3,6 +3,9 @@ import { useI18n } from '../i18n'
 
 const MAX_POINTS = 30
 
+const POLL_INTERVAL = 5000
+const MAX_IDLE_POLLS = 3
+
 function MiniGraph({ data, max, color, label, unit }) {
   if (!data || data.length < 2) return <div className="dash-graph-empty">collecting...</div>
   const m = max || Math.max(...data, 1)
@@ -71,8 +74,19 @@ export default function Dashboard({ api, refreshRef }) {
   useEffect(() => {
     loadData()
     if (refreshRef) refreshRef.current = loadData
-    const iv = setInterval(loadData, 5000)
-    return () => clearInterval(iv)
+    let active = true
+    let idleTicks = 0
+    const iv = setInterval(() => {
+      const hidden = document.querySelector('.dash-grid')?.closest('.tab-hidden')
+      if (hidden) {
+        idleTicks++
+        if (idleTicks >= MAX_IDLE_POLLS) return
+      } else {
+        idleTicks = 0
+      }
+      if (active) loadData()
+    }, POLL_INTERVAL)
+    return () => { active = false; clearInterval(iv) }
   }, [loadData, refreshRef])
 
   const minimax = (quota || []).find(p => p.name === 'minimax')