fix(settings): give the Layout tab its own title instead of Appearance's

[lucletoffe]

What

Settings → Layout renders the header Appearance / Customize the look and feel of your webmail — byte for byte what Settings → Appearance renders. LayoutSettings reads every string from the settings.appearance namespace, including the section title, so once you are on the page there is nothing telling the two tabs apart.

Why it matters

Real case that prompted this: looking for Unified Mailbox → All mail (to make search span every folder). You open Appearance, the page says "Appearance", you find theme / font size / density / avatar toggles, and you conclude the setting does not exist. It is one tab below, under an identical heading.

The sidebar highlight is the only cue, and it is easy to miss on a wide screen where the two tabs sit next to each other under the same APPEARANCE group.

Fix

Only the section header moves to a new settings.layout namespace:

-    <SettingsSection title={t('title')} description={t('description')}>
+    <SettingsSection title={tLayout('title')} description={tLayout('description')}>

Deliberately minimal:

• every item on the page keeps its settings.appearance key, so no existing translation in any of the 30+ locales is invalidated;
• only en/common.json gains the two new strings — locales without settings.layout fall back to English via mergeMessages, so nothing renders a raw key like settings.layout.title.

Checks

• npx tsc --noEmit — clean
• npx eslint components/settings/layout-settings.tsx — clean
• npx vitest run on the neighbouring settings/store suites — 12 passed