The adaptive limiter only covered batch (folder) operations. Single-file
uploads — upload-on-save and ClaudeSync's per-file dispatch — went through
with_retry with retry_count defaulting to 0, so the first rate-limit
rejection failed immediately and popped a modal. With many concurrent files,
dozens of identical 'Connection lost' dialogs stacked up.

- with_retry now applies the same transient-error retry floor (4) as the
  batch path, with exponential backoff + jitter and reconnect between tries.
- upload-on-save is now wrapped in with_retry too (was not retried at all).
- New is_user_actionable() splits errors: auth/host-key/config always show a
  modal (user must fix them); transient connection/timeout/rate-limit errors
  go to the panel + status bar, with at most one modal per 20s cooldown.
- _report_error() centralizes this routing for all single-file operations.