feat(workspaces): paired local/remote folder bookmarks

[Orinks]

Summary

Add workspace bookmarks that save the current local folder and current remote folder as a named pair, so a user can return to a matched local/remote location in one step.

Carried over from the Python-era PR #126, 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

• Save the active local folder and active remote folder together under a user-supplied name.
• An accessible Open Workspace dialog lists saved workspaces, keyboard-selectable, with a Remove button.
• Bookmarks persist as non-secret data (workspaces.json) in the normal config directory, so they follow the data folder in portable mode.

Notes for the Rust implementation

• Persistence belongs in portkeydrop-core alongside sites.rs / settings.rs, so it can be unit-tested without a display.
• The 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.
• Opening a workspace changes the remote directory, which is network work and must go through a worker posting an AppEvent, not the UI thread.
• Original Python reference: feat(workspaces): add paired folder bookmarks #126