fix(FN-1207): improve mobile header search behavior

- Replace delayed focus effect with native autoFocus on the mobile search input
- Constrain expanded mobile search positioning and width to prevent overflow on small viewports
- Add Header tests covering mobile search auto-focus and expanded container rendering

packages/dashboard/app/components/Header.tsx

@@ -195,13 +195,6 @@ export function Header({
     return () => document.removeEventListener("keydown", handleKeyDown);
   }, []);
 
-  // Focus input when mobile search opens
-  useEffect(() => {
-    if (isMobileSearchOpen && mobileSearchInputRef.current) {
-      setTimeout(() => mobileSearchInputRef.current?.focus(), 0);
-    }
-  }, [isMobileSearchOpen]);
-
   const handleMobileSearchToggle = useCallback(() => {
     setIsMobileSearchOpen((prev) => !prev);
   }, []);
@@ -335,6 +328,7 @@ export function Header({
                 <Search size={14} className="header-search-icon" />
                 <input
                   ref={mobileSearchInputRef}
+                  autoFocus
                   type="text"
                   placeholder="Search tasks..."
                   value={searchQuery}