Fix: deduplicate localized reply/forward subject prefixes

Replying to a reply produced "Re: Re: foo" (and German used the English
"Re:"/"Fwd:" instead of "AW:"/"WG:"). Four code paths built reply/forward
subjects and only one deduplicated - and only for the English prefix, so
cross-locale threads accumulated chains.

New lib/subject-prefix.ts strips any leading run of reply/forward markers
across ~35 tokens from all supported languages (plus Outlook Re[2]: and
Eudora Re*2: counters), then prepends the locale-appropriate prefix. All four
call sites (composer getInitialSubject, the two page.tsx sites, and the three
pro-tab handlers) now use buildReplySubject/buildForwardSubject. German prefix
corrected to AW:/WG:.

components/email/email-composer.tsx

@@ -10,6 +10,7 @@ import { cn, formatFileSize, formatDateTime, generateUUID } from "@/lib/utils";
 import { debug } from "@/lib/debug";
 import { toast } from "@/stores/toast-store";
 import { sanitizeSignatureHtml } from "@/lib/email-sanitization";
+import { buildReplySubject, buildForwardSubject } from "@/lib/subject-prefix";
 import { emailHooks, contactHooks } from "@/lib/plugin-hooks";
 import type { OutgoingEmail, RecipientSuggestion } from "@/lib/plugin-types";
 import { useAuthStore } from "@/stores/auth-store";
@@ -265,11 +266,9 @@ export function EmailComposer({
   const getInitialSubject = () => {
     if (!replyTo?.subject) return "";
     if (mode === 'forward') {
-      const fwdPrefix = t('prefix.forward');
-      return `${fwdPrefix} ${replyTo.subject.replace(/^(Fwd:\s*|Tr:\s*)+/i, '')}`;
+      return buildForwardSubject(replyTo.subject, t('prefix.forward'));
     } else if (mode === 'reply' || mode === 'replyAll') {
-      const rePrefix = t('prefix.reply');
-      return `${rePrefix} ${replyTo.subject.replace(/^(Re:\s*)+/i, '')}`;
+      return buildReplySubject(replyTo.subject, t('prefix.reply'));
     }
     return "";
   };