fix: strip reply/forward prefixes followed by a full-width colon

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.

lib/__tests__/subject-prefix.test.ts

@@ -10,10 +10,9 @@ describe('stripSubjectPrefixes', () => {
     expect(stripSubjectPrefixes('Re: AW: WG: foo')).toBe('foo');
   });
 
-  it('strips the Outlook [N] counter and Eudora *N counter', () => {
+  it('strips the Outlook [N] and Eudora *N counters', () => {
     expect(stripSubjectPrefixes('Re[2]: foo')).toBe('foo');
     expect(stripSubjectPrefixes('Re*3: foo')).toBe('foo');
-    expect(stripSubjectPrefixes('Re*: foo')).toBe('foo');
   });
 
   it('is case-insensitive and idempotent', () => {
@@ -21,47 +20,37 @@ describe('stripSubjectPrefixes', () => {
     expect(stripSubjectPrefixes(stripSubjectPrefixes('RE: Re: foo'))).toBe('foo');
   });
 
-  it('strips a Cyrillic reply token', () => {
+  it('strips a Cyrillic token and an ASCII-colon Chinese token', () => {
     expect(stripSubjectPrefixes('Ответ: foo')).toBe('foo');
-  });
-
-  it('strips a Chinese token followed by an ASCII colon', () => {
     expect(stripSubjectPrefixes('回复: foo')).toBe('foo');
   });
 
-  it('CHARACTERISATION: does NOT strip a token followed by a full-width colon', () => {
-    // The colon in the regex is ASCII ":"; a full-width "：" (U+FF1A), as some
-    // CJK mail clients emit, is left untouched. Likely a bug — see follow-ups.
-    expect(stripSubjectPrefixes('回复：foo')).toBe('回复：foo');
+  it('strips a token followed by a full-width colon (CJK clients)', () => {
+    expect(stripSubjectPrefixes('回复：foo')).toBe('foo');
+    expect(stripSubjectPrefixes('回覆：foo')).toBe('foo');
+    expect(stripSubjectPrefixes('Re：foo')).toBe('foo');
   });
 
-  it('does NOT strip a bare single-letter "R:" (would eat real subjects)', () => {
+  it('still does not strip a bare single-letter "R:"', () => {
     expect(stripSubjectPrefixes('R: budget 2024')).toBe('R: budget 2024');
   });
 
-  it('returns "" for empty / null / undefined', () => {
+  it('returns "" for empty / null / undefined and leaves clean subjects alone', () => {
     expect(stripSubjectPrefixes('')).toBe('');
     expect(stripSubjectPrefixes(null)).toBe('');
     expect(stripSubjectPrefixes(undefined)).toBe('');
-  });
-
-  it('leaves a prefix-free subject untouched', () => {
     expect(stripSubjectPrefixes('foo')).toBe('foo');
   });
 });
 
 describe('buildReplySubject / buildForwardSubject', () => {
-  it('replaces an existing prefix chain with the given prefix', () => {
-    expect(buildReplySubject('AW: WG: foo', 'Re:')).toBe('Re: foo');
+  it('replaces a prefix chain (incl. a full-width colon) with the given prefix', () => {
+    expect(buildReplySubject('回复：foo', 'Re:')).toBe('Re: foo');
     expect(buildForwardSubject('Re: foo', 'Fwd:')).toBe('Fwd: foo');
   });
 
-  it('prepends the prefix to a prefix-free subject', () => {
+  it('prepends to a clean subject and returns the bare prefix for empty input', () => {
     expect(buildReplySubject('foo', 'AW:')).toBe('AW: foo');
-  });
-
-  it('returns just the bare prefix for an empty subject', () => {
     expect(buildReplySubject('', 'AW:')).toBe('AW:');
-    expect(buildForwardSubject(null, 'Fwd:')).toBe('Fwd:');
   });
 });

lib/subject-prefix.ts

@@ -64,8 +64,10 @@ function buildPrefixRegex(tokens: string[]): RegExp {
   // Sort by length DESC so longer tokens (e.g. "Пересл") win over their
   // shorter prefixes (e.g. "Пер") during alternation matching.
   escaped.sort((a, b) => b.length - a.length);
+  // Accept both the ASCII colon and the full-width colon "：" (U+FF1A) that CJK
+  // mail clients emit after a localized prefix (e.g. "回复：foo").
   return new RegExp(
-    `^\\s*(?:${escaped.join("|")})(?:\\[\\d+\\]|\\*\\d*)?\\s*:\\s*`,
+    `^\\s*(?:${escaped.join("|")})(?:\\[\\d+\\]|\\*\\d*)?\\s*[:\\uFF1A]\\s*`,
     "i",
   );
 }