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/App.jsx

@@ -92,16 +92,6 @@ export default function App() {
     config: [],
   }), [layout, t])
 
-  const renderContent = () => {
-    switch (activeTab) {
-      case 'dash': return <Dashboard api={api} refreshRef={dashRefreshRef} />
-      case 'studio': return <Studio api={api} />
-      case 'shell': return <Shell api={api} />
-      case 'config': return <Config api={api} />
-      default: return null
-    }
-  }
-
   return (
     <div className="app-layout">
       <header className="header">
@@ -143,8 +133,11 @@ export default function App() {
         </span>
       </header>
 
-      <main className="content fade-in" key={`${activeTab}-${TABS.length}`}>
-        {renderContent()}
+      <main className="content">
+        <div className={activeTab === 'dash' ? '' : 'tab-hidden'}><Dashboard api={api} refreshRef={dashRefreshRef} /></div>
+        <div className={activeTab === 'studio' ? '' : 'tab-hidden'}><Studio api={api} /></div>
+        <div className={activeTab === 'shell' ? '' : 'tab-hidden'}><Shell api={api} /></div>
+        <div className={activeTab === 'config' ? '' : 'tab-hidden'}><Config api={api} /></div>
       </main>
 
       <footer className="statusbar">