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:.

locales/de/common.json

@@ -578,8 +578,8 @@
     "subject_label": "Betreff:",
     "file_size_kb": "KB",
     "prefix": {
-      "forward": "Fwd:",
-      "reply": "Re:"
+      "forward": "WG:",
+      "reply": "AW:"
     },
     "no_subject": "(Kein Betreff)",
     "unknown_sender": "Unbekannt",