feat(settings): turn Appearance into a hub of per-module subpages

Appearance ("Darstellung") stacked every module's view and ordering
settings on one long page. Split it into a hub that lists the modules as
navigable rows, each opening its own /settings/layout/<module> subpage
with a back link — the per-module settings move verbatim, no restyling.
The module routes now redirect to those subpages instead of an in-page
anchor, and the in-app cogs link straight to them.

src/components/dashboard/dashboard-header.tsx

@@ -110,7 +110,7 @@ export function DashboardHeader({
           data-testid="dashboard-customize-shortcut"
         >
           <Link
-            href="/settings/dashboard"
+            href="/settings/layout/dashboard"
             aria-label={t("dashboard.customizeDashboard")}
             title={t("dashboard.customizeDashboard")}
           >