feat: 添加标题长度校验防止小红书API拒绝 (#53)

- 新增go-runewidth依赖用于计算字符显示宽度
- 在PublishContent方法中添加标题长度验证
- 标题超过40个单位时返回"标题长度超过限制"错误
- 中文字符计2个单位，ASCII字符计1个单位
- 确保标题符合小红书API要求：最多20个中文字符或40个ASCII字符

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

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

go.mod

@@ -29,10 +29,12 @@ require (
 	github.com/klauspost/cpuid/v2 v2.2.7 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mattn/go-runewidth v0.0.16 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.2 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/rivo/uniseg v0.2.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.12 // indirect
 	github.com/ysmood/fetchup v0.2.3 // indirect