feat(compare): accessible directory comparison preview

[Orinks]

Summary

Add a read-only directory comparison that shows which entries differ between the current local folder and the current remote folder, with an accessible preview a screen reader user can read through.

Carried over from the Python-era PR #125, which was closed unmerged when the app moved to Rust. The design below is from that PR; the implementation has to be redone against the Rust crates.

Design from the original PR

• A comparison engine that classifies each entry as local-only, remote-only, same, or different (by size and, where available, modification time).
• Exposed as Compare Directories on the View menu with Ctrl+Shift+C.
• Results shown in a read-only preview dialog rather than acting on files; the comparison never transfers or deletes anything.

Notes for the Rust implementation

• The comparison logic belongs in portkeydrop-core so it is tested against RemoteFile lists without a display.
• Listing the remote side is network work and must run on a worker that posts an AppEvent; the dialog is populated when the result arrives.
• The result dialog goes under crates/portkeydrop/src/ui/dialogs/. Every control needs an explicit accessible name via set_name, and the list needs a StaticText immediately before it so NVDA reads it as the list's name.
• Original Python reference: feat(compare): add accessible directory comparison preview #125