feat(settings): WhatsApp brand mark on the WhatsApp settings page (#1418)

The page had no visual signal that it configures a third-party channel,
so it read like any other Accounted setting. Adds an inline-SVG WhatsApp
mark next to the section title and on the button that opens WhatsApp
(replacing the generic lucide chat bubble, which was standing in for a
logo it is not).

Inline SVG rather than a bundled asset: no network request, scales, and
survives the CSP. SettingsSectionHeader gains an optional `mark` slot;
every other settings tab is untouched and stays mark-less on purpose, so
the rail does not turn into a sticker album. The green is the one place
brand colour appears in settings, which matches the design rule that
colour belongs to actors rather than chrome.

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/settings/sections/WhatsAppSettingsContent.tsx

@@ -2,6 +2,7 @@
 
 import { useTranslations } from 'next-intl'
 import { WhatsAppLinkPanel } from '@/components/extensions/general/WhatsAppLinkPanel'
+import { WhatsAppMark } from '@/components/extensions/general/WhatsAppMark'
 import { SettingsSectionHeader } from '@/components/settings/SettingsRows'
 
 export function WhatsAppSettingsContent() {
@@ -10,7 +11,11 @@ export function WhatsAppSettingsContent() {
 
   return (
     <div>
-      <SettingsSectionHeader title={tNav('whatsapp')} intro={tIntro('whatsapp')} />
+      <SettingsSectionHeader
+        title={tNav('whatsapp')}
+        intro={tIntro('whatsapp')}
+        mark={<WhatsAppMark size={26} />}
+      />
       <WhatsAppLinkPanel />
     </div>
   )