feat(ui): icons on every action, and rename Flag to Important

Items 56 and 57, done together because both touch the action registry.

56. The complaint was inconsistency, not absence: eight of twenty-four
actions had themed icons, so two adjacent entries in one menu disagreed
and the toolbar laid out an empty slot for the other sixteen. The
themeIcons table now covers them all. The fifteen names added were
probed against a live icon theme first rather than taken from the
freedesktop spec on faith, and the existing null-icon guard still lets a
theme that lacks one fall back to text.

The second half of the note asked that buttons honour the desktop's
"Icon only" setting. They could not: the hardcoded setToolButtonStyle
overrode it whatever the user had chosen. It now reads
SH_ToolButtonStyle. Dropping the call entirely was tried and rejected,
since a bare QToolBar defaults to ToolButtonIconOnly rather than to the
platform hint, which ignores the setting just as thoroughly the other
way. This is a visible change: on a desktop set to "Icon only" the
toolbar now shows icons without text.

57. "Important" over "Starred", the user's pick; the Message menu
already has "Mark &spam", so "Starred" would have needed an accelerator
from inside the word. Changed the action text, its status tip, the undo
description and the star column's tooltip, which still read "Flagged".

The tag stays `flagged`. It is wire format that neomutt, the user's
saved queries and ThreadSummary::isFlagged() all read, and following the
label through to the tag would rewrite the mail store and desynchronise
every other tool over the same Maildir. The action name stays `flag`
too, since that is the key users write in [keys].

Four tests. everyActionCarriesAnIcon names every action missing one and
guards against passing on an empty list; it reported all sixteen before
the change. theImportantActionStillWritesTheFlaggedTag asserts on the
tag the model actually received, and mutating it to `important` fails
that test plus two pre-existing held-edit tests.

Also adds the changelog entry for the cron-sync indicator fix, which the
commit that made it omitted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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