Send folders (preserve directory structure)

[mxmgorin]

Description

Let the file browser select a folder and send it recursively, recreating its directory layout on the receiver. Today the browser selects individual files only (dirs just navigate), and each file is sent with a flat basename — so sending a gbc/ folder of ROMs means hand-picking every file and losing the structure.

Plan

1. Browser: allow selecting a directory, distinct from entering it (src/overlay/browser.rs; A currently navigates on is_dir). A selected folder is walked recursively, adding files with their path relative to the folder's parent (e.g. gbc/game.gbc).
2. Outbound: carry that relative path in FileMeta.file_name (src/transfer/outbound.rs:96 uses the basename today) — LocalSend conveys folder layout via fileName.
3. Receiver: recreate subdirectories under the save dir. This reverses today's deliberate flattening (src/transfer/inbound.rs:72) — replace with a subpath-aware, traversal-safe join.
4. Selection totals + progress reflect the recursive file set.

Notes

• Path-traversal safety is the crux: file_name is attacker-controlled network input — sanitize each component, drop ./../empties, reject absolute/drive-relative paths, and keep the dest provably under the save root (keep an assert like the current one).
• Interop: verify structure round-trips with the official LocalSend app both ways.
• Save-routes interaction: per-extension routing (src/transfer/route.rs) is for loose files; for a folder send, preserving the sent structure under save_dir should win (decide + document).
• Large/deep folders: recursive walk + per-file slot/token setup must stay responsive.
• Empty dirs aren't representable in per-file metadata — they'll be skipped.