feat(compose): report autosave state in a status bar

Autosave worked and said nothing on success. The only feedback was
m_banner, which is the failure channel and whose persistence is
load-bearing for the quit path, so success got its own channel rather
than sharing one.

The fix is a funnel, not a label. m_dirty had seven writers, four of
which clear it and only two of those are a save: the constructor clears
it because seeding is not an edit, and the send handler clears it
because the message is gone. A cue hung off saveDraftNow() would have
been silently wrong in both. setDirty() is the only writer now, and it
refreshes the status cue and setWindowModified() together so neither
display can drift from the flag.

The age line needs a tick of its own, since it moves with no edit to
drive it. Five seconds against a label that reads in tens of them.

Two defects found by probing rather than by reading. The %n plural
rendered as "2 minute(s) ago" for every English user, because Qt picks a
plural form only when a translation supplies the forms and there is no
English .ts; it uses %1 and "min" now, which Italian substitutes
identically. And the status mark was inside the translatable string,
where a translator could drop it; it is concatenated outside tr().

Presentation reworked after the user looked at it. The first version
reused item 151's yellow ribbon treatment, which reads as a misplaced
widget on a bare status label rather than as a warning, and put both
labels in the permanent widget area, which is the right-hand tray. They
are ordinary status text on the left now.

onlyTheSetterWritesTheDirtyFlag() asserts the funnel structurally, by
reading composewindow.cpp: the first test for the send path called
markClean() directly and a mutation restoring a direct assignment left
the whole suite green. Four mutations now fail. The suite still cannot
see the presentation, which is why that half needed a hand test.

lrelease reports 487 finished, 0 unfinished.

Closes item 160, and unblocks 161.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UUQS6n3cmsFrsjCNmwNtf8

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