fix(mail-overview): parse account sections by walking lines

Two bugs, both found by actually running the shell rather than reading the
code.

shell.qml used Connections without importing QtQuick, so the config failed to
load outright: "Connections is not a type". That one was loud, and it only
affected the temporary verification probe.

The parser was the real bug and it was quiet. Section bodies were matched as
"everything up to the next [", which is wrong for this file: accounts whose
folders are named like [Gmail]/Bozze end their body at that bracket, before
the label line is reached. Three of the five accounts therefore fell back to
displaying their raw key, and nothing reported a problem because falling back
is a legitimate path for an account that genuinely has no label.

Replaced with a line walk. It says what it means, its only state is which
section is open, and it was checked against the real config before being
trusted: all five accounts now resolve both label and colour. An attempted
fix with a lazy quantifier and a lookahead made things worse, dropping every
label, which is the argument for the boring version.

Verified in a running shell: five accounts in config order with their labels,
counts summing exactly to the global notmuch count, no unknowns, and the
config stays loaded for the full timeout rather than exiting silently.

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

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