feat(fm): cross-connection directory transfers (copy + move)

[iret77]

What

Copying/moving a folder between a local and a remote pane (F5/F6) used to silently skip it (N folder(s) skipped (not yet across connections)). Now directories transfer recursively, both directions.

How

• plan_dir_transfer (off the main thread) enumerates the source tree and creates the destination directory structure:
  • Upload (local → remote): walk the local tree with WalkDir (parents before children), mkdir on the remote, collect the files to upload.
  • Download (remote → local): walk the remote tree via list_dir, create_dir_all locally, collect the files to download.
  • Returns the per-file (local, remote) work plus a count of files skipped because they already exist on the destination (matching the single-file cross-connection behaviour; a conflict prompt is a separate follow-up).
• spawn_dir_transfer runs the plan, then spawns one transfer per file through the existing engine — same per-file progress + cancel as before.
• Directory move is safe. Every file is copied, and the whole source tree is removed only once all files land successfully. A batch counter (PendingDirMoveCleanup, keyed by a batch id threaded through spawn_transfer_with_backend) tracks completion; a partial failure keeps the source — never a partial data loss. The batch is registered before spawning so a synchronous (test-mode) completion still counts down correctly.

Verification

• cargo check -p warp — clean, 0 errors/warnings.
• cargo test -p warp --lib sftp_manager — 258 passed, 0 failed, including 5 new tests:
  • plan_dir_transfer upload / download / skip-existing (real local fs + an in-memory backend as the remote side, so paths resolve correctly).
  • directory-move cleanup deletes the source on full success and keeps it on any failure.

Files

app/src/sftp_manager/browser.rs (plan + spawn + batch cleanup + transfer routing), browser_integration_tests.rs (tests).

Follow-ups (still open backlog)

• remote↔remote transfers (relay via a local temp).
• an overwrite prompt on cross-connection conflicts (today they skip existing, consistent with the pre-existing file path).

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