feat(toolbar): add smooth transition animation for AppSwitcher compact toggle

Replace conditional rendering with always-rendered span driven by CSS
max-width + opacity animation. Add time-lock in useAutoCompact to prevent
ResizeObserver flicker during expand animation.

src/components/AppSwitcher.tsx

@@ -1,6 +1,7 @@
 import type { AppId } from "@/lib/api";
 import type { VisibleApps } from "@/types";
 import { ProviderIcon } from "@/components/ProviderIcon";
+import { cn } from "@/lib/utils";
 
 interface AppSwitcherProps {
   activeApp: AppId;
@@ -52,22 +53,28 @@ export function AppSwitcher({
           key={app}
           type="button"
           onClick={() => handleSwitch(app)}
-          className={`group inline-flex items-center gap-2 px-3 h-8 rounded-md text-sm font-medium transition-all duration-200 ${
+          className={cn(
+            "group inline-flex items-center px-3 h-8 rounded-md text-sm font-medium transition-all duration-200",
             activeApp === app
               ? "bg-background text-foreground shadow-sm"
-              : "text-muted-foreground hover:text-foreground hover:bg-background/50"
-          }`}
+              : "text-muted-foreground hover:text-foreground hover:bg-background/50",
+          )}
         >
           <ProviderIcon
             icon={appIconName[app]}
             name={appDisplayName[app]}
             size={iconSize}
           />
-          {!compact && (
-            <span className="transition-all duration-200 whitespace-nowrap">
-              {appDisplayName[app]}
-            </span>
-          )}
+          <span
+            className={cn(
+              "transition-all duration-200 whitespace-nowrap overflow-hidden",
+              compact
+                ? "max-w-0 opacity-0 ml-0"
+                : "max-w-[80px] opacity-100 ml-2",
+            )}
+          >
+            {appDisplayName[app]}
+          </span>
         </button>
       ))}
     </div>

src/hooks/useAutoCompact.ts

@@ -14,12 +14,16 @@ export function useAutoCompact(
 ): boolean {
   const [compact, setCompact] = useState(false);
   const normalWidthRef = useRef(0);
+  const lockUntilRef = useRef(0);
 
   useEffect(() => {
     const el = containerRef.current;
     if (!el) return;
 
     const ro = new ResizeObserver(() => {
+      // During expand animation, ignore resize events to prevent flicker
+      if (Date.now() < lockUntilRef.current) return;
+
       if (!compact) {
         // Cache the total content width in normal mode
         normalWidthRef.current = el.scrollWidth;
@@ -31,6 +35,8 @@ export function useAutoCompact(
         // In compact mode: only recover to normal if
         // available space >= what normal mode needed
         if (el.clientWidth >= normalWidthRef.current) {
+          // Lock out resize events during the expand animation (200ms + 50ms margin)
+          lockUntilRef.current = Date.now() + 250;
           setCompact(false);
         }
       }