fix: dashboard switcher, platform-aware search shortcut, mobile search, static topbar

- Always show dashboard tab strip + add button (was gated behind length > 1)
- Search shortcut now shows Ctrl+K on Windows/Linux, ⌘K on Mac
- Add icon-only search button in topbar for mobile viewports
- Lock html/body overflow and use 100dvh to prevent topbar scrolling off-screen on mobile

src/app/d/[slug]/page.tsx

@@ -93,7 +93,7 @@ export default async function DashboardPage({
 
   return (
     <div className={containerCls}>
-      {userDashboards.length > 1 && (
+      {userDashboards.length >= 1 && (
         <div className="flex items-center gap-1 mb-4 overflow-x-auto">
           {userDashboards.map((d) => (
             <DashboardTab key={d.id} slug={d.slug} name={d.name} />