frontend: fix file attach dropping the picked file (live FileList race)

The file pickers read event.target.files but reset event.target.value = ""
before the file was actually used: in the AI composer the value reset ran
before React's deferred setState updater read the (now-empty) live FileList,
so no chip appeared; in Messages the reset ran before the length check, so it
returned early. Snapshot the files into a plain array synchronously, before the
reset. This is why attaching seemed to fail (no badge) — especially while typing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/components/chat/chat-input.tsx

@@ -85,12 +85,17 @@ export function ChatInput({
 
   const handleFilesSelected = useCallback(
     (event: ChangeEvent<HTMLInputElement>) => {
-      const selected = event.target.files;
+      // Copy the files out NOW: `event.target.files` is a live FileList, and the
-      if (selected && selected.length > 0) {
+      // `event.target.value = ""` reset below empties it. React runs the
-        setFiles((prev) => [...prev, ...Array.from(selected)]);
+      // functional setState updater during a later render, so reading the
-      }
+      // FileList inside the updater would see it already cleared (no file added,
+      // no chip). Snapshot to a plain array first.
+      const picked = Array.from(event.target.files ?? []);
       // Reset so picking the same file again still fires onChange.
       event.target.value = "";
+      if (picked.length > 0) {
+        setFiles((prev) => [...prev, ...picked]);
+      }
     },
     []
   );

frontend/components/messages/messages-view.tsx

@@ -283,12 +283,15 @@ export function MessagesView() {
 
   // Open the file picker; on select, upload each and stage it.
   const onPickFiles = async (event: ChangeEvent<HTMLInputElement>) => {
-    const files = event.target.files;
+    // Snapshot before resetting: `event.target.files` is a live FileList that
+    // `event.target.value = ""` empties, so reading it afterwards (or in a later
+    // tick) yields nothing.
+    const picked = Array.from(event.target.files ?? []);
     event.target.value = "";
-    if (!files?.length) return;
+    if (picked.length === 0) return;
     setUploading(true);
     try {
-      for (const file of Array.from(files)) {
+      for (const file of picked) {
         if (file.size > MAX_ATTACHMENT_BYTES) {
           notify.error(
             t("messages.attach.tooLargeTitle"),