fix: batch merge of bounty fixes (PRs #2743-#2755)

Merged fixes for multiple issues:
- #2743: Stdout/Stderr interleaving order - ordered reading
- #2744: CWD deleted during execution - validation added
- #2745: --retry ignores HTTP 429 Retry-After header - support added
- #2746: Hangs on Content-Length mismatch - read timeout added
- #2749: scrape doesn't persist cookies - cookie store enabled
- #2750: scrape no HEAD method support - --method flag added
- #2751: import cryptic Base64 error - clear messages added
- #2752: Panics on malformed API response - EmptyResponse handling
- #2754: Config corruption on kill during write - atomic writes
- #2755: Busy loop on non-blocking stdin - backoff polling

Based on PR #287 (fix/bounty-batch-2743-2755)

Author: factorydroid

cortex-cli/src/import_cmd.rs

@@ -3,6 +3,7 @@
 //! Imports a session from a portable JSON format (exported or shared).
 
 use anyhow::{Context, Result, bail};
+use base64::Engine;
 use clap::Parser;
 use std::collections::HashSet;
 use std::path::PathBuf;