feat: right-click context menus in the file-tree sidebar

[fredclausen]

What

Adds right-click context menus to the workspace sidebar (first of two PRs; the editor context menu follows).

File rows: Open, Copy path, Copy relative path, Reveal in file manager, Rename, Delete (move to trash).

Folder rows (and the empty tree area, which targets the workspace root): New file, New folder, Copy path, Copy relative path, Reveal in file manager, Rename, Delete (move to trash).

How

• New fs_ops module with rename, create_file, create_dir, and move_to_trash, each returning a typed FsError (added to error.rs). Kept out of the UI loop so the validation and disk effects are unit-tested.
• Name entry (rename / new file / new folder) uses a modal dialog mirroring the existing unsaved-changes prompt; invalid names and clobber attempts surface inline and keep the modal open.
• Delete goes to the OS trash (recoverable) via the trash crate (pinned 5.2.6, default features off to avoid pulling in chrono).
• Editor/workspace reconciliation:
  • Renaming a file or directory repoints any open tab whose path lived at or under it, and migrates matching entries in the workspace expanded-folder set.
  • Trashing a path closes the tabs it backed and prunes now-stale expanded entries.
• Menu choices are collected into a deferred TreeAction and applied after the tree borrow ends, matching the existing MenuAction pattern.

Notes

• Reveal in file manager shells out to the platform opener (xdg-open / open / explorer) on the containing directory; failures are logged, not fatal.
• The trash crate is the one new dependency.

Verification

cargo fmt --check, cargo clippy --all-targets --all-features -D warnings, cargo test --all (101 pass, +13 new across fs_ops and app), and cargo machete all green in the Nix dev shell. Release build compiles.

Follow-up

Editor context menu (Cut/Copy/Paste/Select All + Find/Find & Replace/Save) in a second PR \u2014 egui's TextEdit ships no built-in context menu, so that one adds rather than replaces.