fix(ui): the message pane follows the desktop theme

The stylesheet hardcoded #bbb, #555, #000, #666, #ddd and #4a6f8a, and
set no background at all, so on a dark desktop plain-text mail rendered
as black on white inside a dark window and the web view's own default
showed through.

The colours now come from a Palette struct derived from QPalette, and
are passed into the builder rather than read from qApp inside it, so
the stylesheet can be tested against a known palette with no running
application. Base and Text rather than Window and WindowText: the pane
is a content surface like a text edit, and on many themes those differ.

The secondary colours are blends of text and background, not fixed
greys. That is the part that makes it work both ways round, since a
#555 chosen to read as subtle on white is nearly invisible on #2b2b2b.
The quote colour keeps its hue, because "this is quoted" is carried by
being a different colour rather than a dimmer one, but is pulled toward
the background so it stays readable instead of glowing on dark.

A sender's own HTML is deliberately left alone, and a test asserts that
so it cannot drift: rewriting a sender's styling would break layouts
that depend on it, and a newsletter setting a white background is
entitled to stay white. This themes the plain-text render and the
chrome around messages, nothing more.

MessageView passes its own widget palette rather than the
application's, since a style sheet or a themed parent can give the pane
different colours from qApp, and re-renders on PaletteChange: the
document's colours are baked into its stylesheet at build time, so
unlike a widget it does not restyle itself when the desktop theme
changes.

The load-bearing test asserts the negative, that no hex colour appears
in the style block which the palette did not supply. A test checking
only that the palette's colours are present passes with a leftover
literal still there, and one leftover literal is the whole defect.
Confirmed by mutation.

Closes item 12.

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

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