Simplify i18n: remove old react-intl artifacts, rename formatMessage to t, replace FormattedMessage with t.rich().

- Rewrite messages.ts to plain string key maps (remove MessageDescriptor)
- Rewrite useMessages hook to expose t from useTranslations() directly
- Rename formatMessage → t across 193 consumer files
- Replace custom FormattedMessage component with next-intl t.rich()
- Update 52 language files to use rich text tags (<b>, <a>)
- Remove all direct imports from @/components/messages in favor of useMessages()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/(main)/settings/preferences/PreferencesPage.tsx

@@ -7,12 +7,12 @@ import { useMessages } from '@/components/hooks';
 import { PreferenceSettings } from './PreferenceSettings';
 
 export function PreferencesPage() {
-  const { formatMessage, labels } = useMessages();
+  const { t, labels } = useMessages();
 
   return (
     <PageBody>
       <Column gap="6">
-        <PageHeader title={formatMessage(labels.preferences)} />
+        <PageHeader title={t(labels.preferences)} />
         <Panel>
           <PreferenceSettings />
         </Panel>