feat(desktop): edit a vault item — change a value, remove a field, rename it - #25

Summary

An item's fields were write-once in the UI: you could add, reveal, copy and never change. So rotating a password meant deleting the item. This adds the missing three: edit a value, remove a field, rename the item.

No new IPC or vault surface — the database already supported all of it, only the UI didn't call it. Editing is set_field with the same name and purpose, which the procedure resolves as ON CONFLICT (item_id, name) DO UPDATE, so the row is replaced in place and the fields trigger keeps the previous value in the item's password history:

startEdit = async (field) => setEditing({           // reveals to edit
  name: field.name,
  value: revealed[field.name] ?? await dcrypt.fields.reveal(item.id, field.name),
})
saveEdit = (field) =>
  dcrypt.fields.set(item.id, field.name, field.purpose, editing.value, field.concealed)

Carrying field.purpose and field.concealed through matters: dropping either would silently turn a totp_seed into text (breaking the code display) or unconceal a password.

Editing takes over the row — reveal/copy hide while it does, so there's no way to copy a half-typed value — and Esc abandons, Enter saves. Removing a field is behind a confirm that says plainly it is not the trash and cannot be undone, unlike an item. The title is click-to-rename in place, Esc to abandon.

Link to Devin session: https://app.devin.ai/sessions/04636534e07048089ffb6b78142e12cd
Requested by: @pyramation