feat: rename and move in file manager (#630)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)

This adds renaming and moving files to the File Manager

* **What changes are included?**

New `/move` and `/rename` endpoints, and corresponding modals and icons
added. Uses the `file.rename()` function, after sanity checking.

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).


Fixes #559, #661, #663. Only touches the File Manager, so low risk of
affecting other systems.

Simpler than #619, at the cost of not migrating the cache of renamed
books.

<img width="870" height="437" alt="image"
src="https://github.com/user-attachments/assets/73e0e750-dfc8-48e0-a7a6-9694470b7ded"
/>
<img width="575" height="318" alt="image"
src="https://github.com/user-attachments/assets/38c5fb19-c38a-436b-b3ad-75c1be7375ab"
/>
<img width="574" height="293" alt="image"
src="https://github.com/user-attachments/assets/1d2a2403-765d-473f-8c4f-c6968e9bbfeb"
/>


---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES**_

I used Codex for the implementation itself, and then carefully reviewed
the code myself. As this is a simple change and only to the webserver,
it is low risk.

src/network/CrossPointWebServer.h

@@ -97,5 +97,7 @@ class CrossPointWebServer {
   void handleUpload(UploadState& state) const;
   void handleUploadPost(UploadState& state) const;
   void handleCreateFolder() const;
+  void handleRename() const;
+  void handleMove() const;
   void handleDelete() const;
 };