Truncate long MIME parse error messages (#202)

## Summary

- Cap `FirstLine()` output at 200 runes using the existing
`TruncateRunes` helper
- Prevents enmime's malformed-MIME error messages (which embed the full
email content as a single line) from flooding console output during sync
- Uses rune-safe truncation to avoid splitting multi-byte UTF-8
characters in non-ASCII error text

## Test plan

- [x] Existing `TestFirstLine` tests pass
- [x] New test cases: long ASCII lines, exact boundary (200 runes), long
first line of multi-line, and Unicode truncation safety

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: wesm

internal/textutil/encoding.go

@@ -136,13 +136,14 @@ func TruncateRunes(s string, maxRunes int) string {
 	return string(runes[:maxRunes-3]) + "..."
 }
 
-// FirstLine returns the first line of a string.
-// Useful for extracting clean error messages from multi-line outputs.
-// Leading newlines are trimmed before extracting the first line.
+// FirstLine returns the first line of a string, capped at 200 runes.
+// Useful for extracting clean error messages from multi-line outputs
+// where the first line itself may be excessively long (e.g. enmime
+// includes malformed MIME content in its error messages).
 func FirstLine(s string) string {
 	s = strings.TrimLeft(s, "\r\n")
 	if idx := strings.Index(s, "\n"); idx >= 0 {
-		return s[:idx]
+		s = s[:idx]
 	}
-	return s
+	return TruncateRunes(s, 200)
 }

internal/textutil/encoding_test.go

@@ -486,6 +486,10 @@ func TestFirstLine(t *testing.T) {
 		{"leading carriage return", "\r\nSecond", "Second"},
 		{"mixed leading newlines", "\r\n\n\rThird", "Third"},
 		{"only newlines", "\n\n\n", ""},
+		{"long line truncated", strings.Repeat("x", 250), strings.Repeat("x", 197) + "..."},
+		{"exactly 200 runes", strings.Repeat("y", 200), strings.Repeat("y", 200)},
+		{"long first line of multi", strings.Repeat("z", 250) + "\nSecond", strings.Repeat("z", 197) + "..."},
+		{"unicode truncation safe", strings.Repeat("é", 250), strings.Repeat("é", 197) + "..."},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {