feat(ui): Add rename/delete category actions in the sidebar

[aronovgj]

This introduces a per‑category actions menu (three‑dot button) in the left navigation.

Features:

• Added per‑category action menu (three dots) in the sidebar.
• Categories can be renamed inline directly from the menu.
• Categories can be deleted from the menu.
• Deleting a category with notes asks for confirmation.
• Deleting a category removes all notes in that category.
• New categories can be created inline (focused rename field immediately).
• Dragging a note onto “New category” creates a category and moves the note there.

Closes #1397
Closes #1120
Probably closes #1669
https://github.com/aronovgj/notes/blob/98e04a03435a384c557e6004a6c9c1cae6a672e4/src/components/NoteItem.vue#L299

This is how it would look:

[JuliaKirschenheuter]

@marcoambrosini In my opinion it does make sense and makes usibility easier around using notes app. What would you say?