feat (UI): drag & drop notes to categories

[aronovgj]

Summary
This PR adds drag & drop support to the Notes UI so notes can be moved between categories directly.

What changed

• Notes are now draggable (when not read-only).
• Dropping a note onto a category in the left sidebar updates the note’s category via the existing setCategory() API call.
• Dropping a note onto “All notes” removes its category (moves it to uncategorized).
• Sidebar items show a visual highlight while a draggable note is hovered over them.

Files touched

• NoteItem.vue: sets drag payload (note id) on drag start.
• CategoriesList.vue: implements drag-over/drop handlers for categories and “All notes”, and hover styling.

First PR, please let me know if I messed something up.