Address remaining review items: L4/M1 + L2/L5 cleanup

- L4: sync_bedrock_credentials (renamed from write_bedrock_static_
  credentials) now also clears a stale ~/.aws/credentials when the
  project no longer uses static-credential Bedrock, so static keys don't
  linger unused in the persistent home volume after switching backends.
  Skipped when /tmp/.host-aws is mounted (host-managed ~/.aws). HOME is
  also set explicitly on the exec env for robustness.
- M1: the Backup button now has a tooltip and the success toast notes
  that the archive includes MCP/config which may contain MCP-embedded
  API keys (OAuth tokens are excluded) — keep it private.
- L2: backup now uses async file IO (tokio::fs::File + AsyncWriteExt,
  tokio::fs::remove_file) instead of blocking std::fs between awaits;
  dropped-file reads use tokio::fs::metadata/read.
- L5: upload_host_file_to_terminal explicitly `mkdir -p`s
  /tmp/triple-c-drops instead of relying on Docker's tar extractor to
  create the parent dir.

Verified L4 cleanup guard, L5 mkdir, async IO, and exit-code paths
against real containers. 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 +196,7 @@ 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 (${mb} MB)`);
+      setProgressMsg(`Backup saved (${mb} MB). Note: includes MCP/config — may contain MCP API keys. Keep it private.`);
     } catch (e) {
       setError(String(e));
     } finally {
@@ -531,7 +531,12 @@ export default function ProjectCard({ project }: Props) {
                 <ActionButton onClick={handleOpenTerminal} disabled={loading} label="Terminal" accent />
                 <ActionButton onClick={handleOpenBashShell} disabled={loading} label="Shell" />
                 <ActionButton onClick={() => setShowFileManager(true)} disabled={loading} label="Files" />
-                <ActionButton onClick={handleBackup} disabled={loading || backingUp} label={backingUp ? "Backing up…" : "Backup"} />
+                <ActionButton
+                  onClick={handleBackup}
+                  disabled={loading || backingUp}
+                  label={backingUp ? "Backing up…" : "Backup"}
+                  title="Downloads /workspace plus a sanitized home config (MCP servers, settings, skills). OAuth tokens are excluded, but MCP server configs may embed their own API keys/tokens — keep the archive private."
+                />
               </>
             ) : (
               <>
@@ -1221,12 +1226,14 @@ function ActionButton({
   label,
   accent,
   danger,
+  title,
 }: {
   onClick: (e?: React.MouseEvent) => void;
   disabled: boolean;
   label: string;
   accent?: boolean;
   danger?: boolean;
+  title?: string;
 }) {
   let color = "text-[var(--text-secondary)] hover:text-[var(--text-primary)]";
   if (accent) color = "text-[var(--accent)] hover:text-[var(--accent-hover)]";
@@ -1236,6 +1243,7 @@ function ActionButton({
     <button
       onClick={(e) => { e.stopPropagation(); onClick(e); }}
       disabled={disabled}
+      title={title}
       className={`text-xs px-2 py-0.5 rounded transition-colors disabled:opacity-50 ${color} hover:bg-[var(--bg-primary)]`}
     >
       {label}