feat(mail-overview): account model singleton

Accounts.qml is the data layer for the mail drawer: it parses accounts
from qtmaildir.conf rather than hardcoding them, so adding an account
there needs no edit here. The key runs to the closing bracket, not the
first dot, because at least one configured key contains a dot of its
own; matching only up to the first dot would silently truncate it and
drop that account's mail from every count.

Counting uses the account-<key> notmuch tag, not a path glob. notmuch
deduplicates by message id, so a message delivered to two of the
configured addresses is one message with two file paths: a path glob
would count it under both accounts and the per-account rows would sum
higher than the global total the waybar icon already shows (measured
elsewhere as 102 vs 101). The tag is a property of the message, so it
is singular and the rows always sum to the header exactly.

An unknown count is -1, never 0. notmuch has one failure mode with no
distinct exit status: a rejected query fragment can still return 0
exit and empty output, and an empty string must not become a zero,
which would render as "no unread mail" instead of "count unavailable".
Output is validated as a non-negative integer before being trusted.

Includes a temporary console-log probe in shell.qml, added
deliberately for runtime verification of this model outside the
drawer UI (Task 4). It stays until the user has run and confirmed it
against real notmuch output, then gets removed in a follow-up commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WWL8JYHu7yhAdtx5pU9PMU

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