Let the theme be forced onto mail that styles itself

Appearance gained "Apply the theme to messages too" some time ago, and it
themes an HTML message only when the message brings no colours of its own.
That predicate is the right default and it almost never passes: one
`color:#FFFFFF` on one button label opts a whole message out, so in real
mail — receipts, shipping notices, anything from a template — the switch
did nothing at all and the reader kept a bright white card on a dark UI.

A second switch, off by default and only meaningful with the first on,
forces the palette over the sender's colours. It cannot be done perfectly,
which is why it is a separate, explicit choice: the same bargain a
dark-reader extension makes.

What it does is tell two kinds of colour apart. A *sheet* the design sits
on — the white 600px wrapper — is neutralised, and a *painted surface* —
a call to action, a footer banner — is kept whole so its label stays
legible on it. Relative luminance decides, at 0.5: white wrappers sit at
1.0, a blue button near 0.09. Only the painted ones are marked, with
data-ihm-keep, and one rule in EMAIL_BASE_CSS neutralises everything else.

Nothing the sender wrote is removed, so the switch is reversible, colours
arriving from a <style> block are covered as well as inline ones, and
print still pins the tokens to ink on white.

The mock grew the message this is about: an outer wrapper on
bgcolor="#ffffff", a <style> block, a coloured button, a grey footer.
Without one, neither the bug nor the fix could be seen.

Verified in a browser against the mock: with only the first switch on the
card is still white; with both, the wrapper computes to transparent, body
text follows the theme, and the button keeps white-on-blue. Two surfaces
marked, which are the two the message paints.

Closes #290

web/src/locales/de.ts

@@ -518,6 +518,8 @@ export const catalog: Catalog = {
     "Show labels in the sidebar": "Labels in der Seitenleiste anzeigen",
     "Collapse sidebar to icons": "Seitenleiste auf Symbole verkleinern",
     "Apply the theme to messages too": "Design auch auf Nachrichten anwenden",
+    "Apply it even to mail that styles itself": "Auch auf Mails anwenden, die sich selbst gestalten",
+    "Most marketing and receipt mail sets a colour somewhere, so the setting above leaves nearly all of it on a white card. With this on, the theme is forced over the sender's own colours: backgrounds they laid the message on are dropped, while buttons and coloured banners are kept so their text stays readable. Some mail will not survive it intact, which is why it is separate.": "Fast jede Werbe- oder Beleg-Mail setzt irgendwo eine Farbe, deshalb lässt die Einstellung darüber nahezu alle davon auf einer weißen Karte. Mit dieser Option wird das Design über die Farben des Absenders gelegt: Hintergründe, auf denen die Nachricht liegt, entfallen, während Schaltflächen und farbige Banner erhalten bleiben, damit ihr Text lesbar bleibt. Manche Mail übersteht das nicht unbeschadet – deshalb ist es eine eigene Einstellung.",
     "Swiping": "Wischgesten",
     "Swipe left": "Nach links wischen",
     "Swipe right": "Nach rechts wischen",