Add a right-click context menu to file rows

Renaming, archiving and deleting a file all lived in buttons under the
list, and all of them required selecting the file first. The same
operations are now on the row itself: right-click a file for Rename,
Copy path, Archive and Delete.

The menu records what was picked rather than acting on it, because the
list is drawn inside a closure still borrowing `self` — the same
deferred pattern the click and drag-drop handling already use. Right-
clicking selects the row first, so the menu always acts on the file you
clicked; selecting is a no-op when it is already current, so this cannot
reload the buffer out from under an unsaved edit.

Rename opens a focused dialog with the path pre-filled, and stays open
if the name is rejected so a clash does not cost what was typed;
rename_selected reports success for that. Delete asks first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBWj9TphFMCoh7VHaSRnvQ

README.md

@@ -221,6 +221,14 @@ and give a chapter its own folder of scenes — nesting goes eight levels deep.
   file just as dragging does.
 * **＋ New from template** puts its `untitled-N.md` in the folder of the file you
   are editing, so working inside a part doesn't scatter new files at the top.
+* **Right-click a file** for a menu of the things you can do to it:
+  **📝 Rename…**, **⧉ Copy path**, **🗄 Archive** and **🗑 Delete…**. It acts on
+  the file you clicked — right-clicking selects it first — so you do not have to
+  select a file before reaching for the operation. Rename opens a small dialog
+  with the path already filled in and the box focused; it stays open if the name
+  is rejected, so a clash never costs you what you typed. Delete asks first.
+  These are the same operations as the buttons under the list, which stay where
+  they are.
 
 Folders are skipped when they cannot hold manuscript prose: anything starting
 with a `.` (so `.git` is never walked), plus `target/` and `node_modules/`.