Translate three lists the code was rendering raw

A native speaker reviewing the German catalogue reported strings appearing in
English (#247). Three of the reported areas turned out not to be missing
translations at all: the strings were there, and the code was rendering the
English source instead of asking for one.

The Sieve rule dialog rendered HEADER_CHOICES and HEADER_OPS labels directly.
All sixteen are already in every catalogue -- "Subject" has been "Betreff" in
de.ts all along, which is exactly the inconsistency the reporter noticed
against the Out-of-office page, where it already reads Betreff. Wrapping the
two dropdowns fixes nine languages at once and adds nothing to any catalogue.

The keyboard shortcut panel rendered each binding's group and description
directly. Those are registered in English at the call sites and should stay
that way -- the binding table is data and the English is the catalogue key --
so the panel translates them at render instead. A binding added anywhere is
then translatable without its registrar knowing i18n exists.

The palette grid marks every name translate="no". That is right for ihasmail,
Dracula, Gruvbox, Rosé Pine and Tokyo Night, which are names. "Classic" is an
adjective describing the theme, not a name, so PaletteMeta gains a
`translatable` flag for the one entry that is a word. Flagging the exception
beats dropping the attribute from all six.

No catalogue changes here: the strings the first two need are already present
in all nine. "Classic" needs an entry, which lands with each language.

web/src/lib/palette.ts

@@ -23,10 +23,20 @@ export interface PaletteMeta {
   name: string;
   /** Shown in Settings and in NOTICE; who to credit and under what. */
   credit?: string;
+  /**
+   * Whether the name is a word rather than a name.
+   *
+   * Five of these six are proper names -- ihasmail, Dracula, Gruvbox, Rosé
+   * Pine, Tokyo Night -- and are rendered translate="no" so a page translator
+   * leaves them alone. "Classic" is not a name, it is an adjective describing
+   * the theme, and a German reader should see "Klassisch". Reported by a
+   * native speaker reviewing the German catalogue (#247).
+   */
+  translatable?: boolean;
 }
 
 export const PALETTES: PaletteMeta[] = [
-  { id: "default", name: "Classic" },
+  { id: "default", name: "Classic", translatable: true },
   { id: "ihasmail", name: "ihasmail" },
   { id: "dracula", name: "Dracula", credit: "Dracula Theme (MIT) — dark: Dracula, light: Alucard" },
   { id: "gruvbox", name: "Gruvbox", credit: "gruvbox by morhetz (MIT)" },