fix: retry giveFocus on next animation frame for newly created blocks (#3100)

Fixes #2926

## Problem

Keyboard scrolling (arrow keys) does not work in newly opened blocks
until the user mouse-clicks. This is a race condition in BlockFull's
focus handling.

When `isFocused` becomes true for a newly created block,
`setFocusTarget()` calls `viewModel.giveFocus()`. But the view's DOM
element (terminal, Monaco editor, webview) may not be mounted yet, so
`giveFocus()` returns false and focus falls back to the hidden dummy
`<input>` element — which cannot handle arrow key scrolling.

## Fix

After falling back to the dummy focus element, schedule a
`requestAnimationFrame` callback that retries `viewModel.giveFocus()`.
This gives React one more frame to flush pending renders and mount the
view's DOM, so focus transfers to the real element once it's ready.

## Test Plan

- Open a new terminal block — verify arrow keys scroll immediately
without clicking
- Open a file preview — verify arrow keys scroll the file content  
- Open a webview — verify keyboard scrolling works
- Switch focus between blocks using keyboard shortcuts — verify
scrolling works in each
- `npx tsc --noEmit` passes clean

---------

Signed-off-by: majiayu000 <1835304752@qq.com>

frontend/app/block/block.tsx

@@ -144,6 +144,7 @@ const BlockFull = memo(({ nodeModel, viewModel }: FullBlockProps) => {
     const focusElemRef = useRef<HTMLInputElement>(null);
     const blockRef = useRef<HTMLDivElement>(null);
     const contentRef = useRef<HTMLDivElement>(null);
+    const pendingFocusRafRef = useRef<number | null>(null);
     const [blockClicked, setBlockClicked] = useState(false);
     const blockView = useAtomValue(waveEnv.getBlockMetaKeyAtom(nodeModel.blockId, "view")) ?? "";
     const isFocused = useAtomValue(nodeModel.isFocused);
@@ -156,6 +157,14 @@ const BlockFull = memo(({ nodeModel, viewModel }: FullBlockProps) => {
     const innerRect = useDebouncedNodeInnerRect(nodeModel);
     const noPadding = useAtomValueSafe(viewModel.noPadding);
 
+    useEffect(() => {
+        return () => {
+            if (pendingFocusRafRef.current != null) {
+                cancelAnimationFrame(pendingFocusRafRef.current);
+            }
+        };
+    }, []);
+
     useLayoutEffect(() => {
         setBlockClicked(isFocused);
     }, [isFocused]);
@@ -221,11 +230,21 @@ const BlockFull = memo(({ nodeModel, viewModel }: FullBlockProps) => {
     );
 
     const setFocusTarget = useCallback(() => {
+        if (pendingFocusRafRef.current != null) {
+            cancelAnimationFrame(pendingFocusRafRef.current);
+            pendingFocusRafRef.current = null;
+        }
         const ok = viewModel?.giveFocus?.();
         if (ok) {
             return;
         }
         focusElemRef.current?.focus({ preventScroll: true });
+        pendingFocusRafRef.current = requestAnimationFrame(() => {
+            pendingFocusRafRef.current = null;
+            if (blockRef.current?.contains(document.activeElement)) {
+                viewModel?.giveFocus?.();
+            }
+        });
     }, [viewModel]);
 
     const focusFromPointerEnter = useCallback(