Third review pass: fix tar TOCTOU + transient backup status

- F2: upload_host_file_to_container now reads the dropped file into a Vec
  inside the blocking task and sizes the tar entry from those exact bytes,
  rather than stat-then-stream where a file changing size between the
  stat and the read could desync the tar header and silently corrupt the
  archive. Still runs off the async worker; memory stays bounded by the
  256 MiB drop cap.
- F4: the "Backup saved" confirmation now auto-clears after 8s (guarded
  against clobbering a newer status message) instead of lingering in the
  project card's status line indefinitely.

F1 (claimed AWS CLI regression from empty-env neutralization) was a false
positive: verified against aws-cli 2.35 that an empty AWS_ACCESS_KEY_ID is
treated as absent and botocore falls through to ~/.aws/credentials (the
call reached AWS and returned InvalidClientTokenId for the file's key, not
PartialCredentialsError). No change needed.

cargo check / tsc / vitest all pass.

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

app/src/components/projects/ProjectCard.tsx

@@ -196,7 +196,11 @@ export default function ProjectCard({ project }: Props) {
       setError(null);
       const bytes = await commands.downloadContainerBackup(project.id, hostPath);
       const mb = (bytes / (1024 * 1024)).toFixed(1);
-      setProgressMsg(`Backup saved (${mb} MB). Note: includes MCP/config — may contain MCP API keys. Keep it private.`);
+      const msg = `Backup saved (${mb} MB). Note: includes MCP/config — may contain MCP API keys. Keep it private.`;
+      setProgressMsg(msg);
+      // Auto-clear so the transient confirmation doesn't linger in the card
+      // status; guard against clobbering a newer message (e.g. a later op).
+      setTimeout(() => setProgressMsg((prev) => (prev === msg ? null : prev)), 8000);
     } catch (e) {
       setError(String(e));
     } finally {