feat(messageview): give the notice bars a ground and a severity

The three out-of-band bars in the message pane were plain labels on the
pane's own background, so they read as part of the page they were trying to
interrupt. They now carry a severity, as the user asked for:

- yellow for a warning that explains a limitation and offers nothing to do
  about it (the receive-only ribbon)
- blue for one offering an action (remote content blocked, stale thread),
  with the button moved right of a stretch, where the eye ends up after
  reading the sentence

Each severity carries its own light and dark set rather than one tint
dimmed, and the theme is read off QPalette::Base, the same surface
HtmlBuilder reads, so a bar and the message under it cannot disagree about
which way round the theme is. Every ground states its own text colour: the
palette's may be near-white, which is unreadable on a pale tint.

The blocked row had to become a widget on the way. It was a bare
QHBoxLayout, which has nothing to paint a ground on, and once wrapped, the
six sites that hid its label and button individually had to hide the
wrapper instead or an empty painted strip would have been left behind.

The test asserts on the stylesheet string rather than on pixels, per
CLAUDE.md on rendering probes: an unshown widget offscreen renders nothing,
so a pixel test would pass whatever the code does. It compares the COLOURS
the sheets name rather than the sheets themselves, since each names its own
widget and would differ by that alone.

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