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/components/quick-add-sheet.tsx

@@ -29,8 +29,14 @@ const QUICK_ADD_ICON: Record<string, string> = {
   "file-plus": "note",
 };
 
+function useShortcutLabel() {
+  if (typeof navigator === "undefined") return "⌘K";
+  return /Mac|iPhone|iPad|iPod/.test(navigator.platform) ? "⌘K" : "Ctrl+K";
+}
+
 export function QuickAddSheet() {
   const { sheetOpen, closeSheet, actions } = useQuickAdd();
+  const shortcut = useShortcutLabel();
   const router = useRouter();
   const backdropRef = useRef<HTMLDivElement>(null);
 
@@ -110,7 +116,7 @@ export function QuickAddSheet() {
             }}
           >
             <Sparkles className="size-3" />
-            <span>Pick what to add — or use ⌘K to search.</span>
+            <span>Pick what to add — or use {shortcut} to search.</span>
           </div>
 
           <div className="max-h-[60vh] overflow-y-auto">