docs(backlog): record items 153 to 156 from the notes

Item 138 gave drafts a button and immediately surfaced that they cannot be
edited: DraftStore has a write() and no reader, and no action opens a
composer from an existing message, so a draft renders as ordinary mail and
can never be finished. Filed as 153, a defect rather than an enhancement.

The user's note splits it in two, "double-clicking a draft should open the
editor" and "there's no edit action anywhere". They are one item: the second
is the general case and the first is one route into it.

154 to 156 are the rest of that note's list, all v2 and all unspecified to
different degrees. 154 and 156 are deliberately separate: a read receipt is
the reader's client to honour, a delivery receipt the sending server's, and
whether the latter can be requested at all depends on the send_command.

152 gains the constraint the user added today: a signature is not tied to an
account and is switched from the editor bar, which rules out a plain
[account.*] key as the whole answer.

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