gmacFTP 0.0.17

A bugfix release. Folder downloads from an FTP/SFTP server were broken; this also tightens the path-containment guard that caused it.

• Folder downloads work again. Dragging a folder (or using the arrow button) from a remote pane to a local pane no longer failed with downloaded 0 file(s); skipped N unsafe path(s). The write-boundary containment guard canonicalized the destination folder path, but that folder does not exist until the first file is written — so canonicalize returned NotFound and every file was silently skipped. The guard now resolves the nearest existing ancestor instead, so a not-yet-created destination root is tolerated (single-file downloads were always unaffected).
• Folder downloads no longer error-loop on session-limited FTP servers. Each file in a folder download opens its own FTP session, and shared-hosting servers that cap concurrent sessions would answer the next connect with 421 Too many connections — which, across a large folder, cascaded into a rapid storm of errors. Downloads now retry with escalating backoff (up to ~8 s) when the server is briefly over its session limit, so the previous file's slot has time to release.
• Arbitrarily large folders stream through a small queue (backpressure). A folder download no longer pre-loads every file into a fixed-size queue — that capped at a few hundred files and failed the rest with transfer queue full. The folder loop now streams jobs into the transfer engine one at a time, waiting when the queue is full, so a 10 000-file download flows through with bounded memory and nothing is dropped.
• No more per-file pane flicker during a folder transfer. The remote/local panes used to re-list (and recalculate folder sizes) after every single file finished. They now refresh once, at the end of the whole batch.
• Transfer UI polish. The compact bottom bar now shows the name of the file currently copying (it used to freeze on the initial label during a fast batch). The transfer panel summary leads with overall progress ("127 / 283 done · …"), the header pill shows done/total with proper padding, and the toolbar Transfers button carries a pending-count badge that opens the panel on click.
• Folders now ask before overwriting. Copying a folder whose name already exists at the destination used to merge silently (only single files prompted). Folders now go through the same overwrite dialog — Overwrite merges into the existing folder, Save-as-new renames, Cancel aborts. (Per-file prompts inside a large folder are intentionally not added — a 300-file folder would mean 300 dialogs.)
• Path-containment guard hardened (defense-in-depth). The same guard now rejects any .. traversal component outright, and the lenient canonicalization no longer silently drops a trailing .. (which Path::file_name() hides). No remote exploit was reachable — sanitize_local_rel already strips .. from server-controlled names — but the write-boundary backstop is now self-sufficient rather than relying on the primary defense.
• Clippy is now clean under -D warnings. Factored the three complex static types into aliases, gave try_enqueue a real error type (QueueFull), made the transfer-thread-local a const initializer, and fixed a handful of redundant-closure / unnecessary-ownership lints. The ten wide internal-helper signatures (AppCtx-style argument lists) are explicitly allowed — they are deliberate and noted as known in ci.yml. Pure code-quality; no behavior change.

Verified: release build + 29 unit/integration tests + cargo clippy -D warnings + cargo fmt --check all green.