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-tauri/src/commands/project_commands.rs

@@ -432,11 +432,11 @@ pub async fn start_project_container(
             new_id
         };
 
-        // Refresh Bedrock static/session credentials on every start so rotated
-        // keys are picked up without a full container recreation. No-op for
-        // other backends / auth methods.
-        if let Err(e) = docker::write_bedrock_static_credentials(&container_id, &project).await {
-            log::warn!("Failed to refresh AWS credentials for project {}: {}", project.id, e);
+        // Sync Bedrock credentials on every start: refresh static/session creds
+        // so rotated keys are picked up without a full container recreation, and
+        // clear stale creds when the project no longer uses static-cred Bedrock.
+        if let Err(e) = docker::sync_bedrock_credentials(&container_id, &project).await {
+            log::warn!("Failed to sync AWS credentials for project {}: {}", project.id, e);
         }
 
         Ok(container_id)