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/terminal_commands.rs

@@ -196,7 +196,8 @@ pub async fn upload_host_file_to_terminal(
 ) -> Result<String, String> {
     let container_id = state.exec_manager.get_container_id(&session_id).await?;
 
-    let meta = std::fs::metadata(&host_path)
+    let meta = tokio::fs::metadata(&host_path)
+        .await
         .map_err(|e| format!("Cannot access {}: {}", host_path, e))?;
     if meta.is_dir() {
         return Err(format!("{} is a directory — drop individual files", host_path));
@@ -213,8 +214,9 @@ pub async fn upload_host_file_to_terminal(
         ));
     }
 
-    let data =
-        std::fs::read(&host_path).map_err(|e| format!("Failed to read {}: {}", host_path, e))?;
+    let data = tokio::fs::read(&host_path)
+        .await
+        .map_err(|e| format!("Failed to read {}: {}", host_path, e))?;
 
     let base = std::path::Path::new(&host_path)
         .file_name()
@@ -222,6 +224,14 @@ pub async fn upload_host_file_to_terminal(
         .filter(|s| !s.is_empty())
         .unwrap_or_else(|| "dropped-file".to_string());
 
+    // Ensure the destination directory exists rather than relying on Docker's
+    // archive extractor to create the parent for the uploaded tar entry.
+    crate::docker::exec::exec_oneshot(
+        &container_id,
+        vec!["mkdir".to_string(), "-p".to_string(), "/tmp/triple-c-drops".to_string()],
+    )
+    .await?;
+
     let file_name = format!("triple-c-drops/{}", base);
     state
         .exec_manager