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/globals.css

@@ -490,6 +490,8 @@
   body {
     margin: 0;
     padding: 0;
+    height: 100%;
+    overflow: hidden;
   }
   body {
     @apply bg-background text-foreground;
@@ -521,6 +523,7 @@
   display: grid;
   grid-template-columns: 232px 1fr;
   height: 100vh;
+  height: 100dvh;
   background: var(--paper);
 }
 :where(html[data-nav="rail"]) .app {