The prefix-stripping regex only matched an ASCII ":", so a localized
prefix from a CJK mail client (e.g. "回复：foo", using the full-width
colon U+FF1A) was left in place. On reply this caused the user's own
prefix to be stacked on top, growing the subject chain.
Accept both ":" and "：" after the prefix token. Adds tests.
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:.
