Surface upload failures instead of silently dropping the files (#1425)

uploadPending() read `data.files` from /api/upload without checking `res.ok`, so
a non-OK response (429 rate limit, 413 too large, …) was swallowed: the pending
files vanished and the chat sent with no attachments and no feedback — part of
why the model "didn't even see them" in #1346.

Check res.ok; on failure show the server's reason via a toast and keep the
pending files so the attach strip re-renders for a retry (matching the existing
"restored on error" comment that the code never actually honored).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/js/fileHandler.js

@@ -172,6 +172,17 @@ export async function uploadPending() {
       method: 'POST',
       body: fd
     });
+    if (!res.ok) {
+      // Surface the failure instead of swallowing it. Previously a non-OK
+      // response (e.g. 429 rate limit, 413 too large) was ignored: the files
+      // silently vanished and the chat sent with no attachments, so the model
+      // "didn't even see them" (issue #1346). Show the server's reason and keep
+      // pendingFiles so the strip re-renders for a retry (see finally below).
+      let detail = '';
+      try { const e = await res.json(); detail = e.detail || e.error || ''; } catch (_) {}
+      _showToast('Upload failed' + (detail ? ': ' + detail : ` (HTTP ${res.status})`));
+      return [];
+    }
     const data = await res.json();
     uploaded = (data.files || []);
     pendingFiles = [];          // clear only on success