fix(form): label无法修改字号

[xiaoyatong]

🤔 这个变动的性质是？

• 新特性提交
• 日常 bug 修复
• 站点、文档改进
• 演示代码改进
• 组件样式/交互改进
• TypeScript 定义更新
• 包体积优化
• 性能优化
• 功能增强
• 国际化改进
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他改动（是关于什么的改动？）

🔗 相关 Issue

💡 需求背景和解决方案

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• 文档已补充或无须补充
• 代码演示已提供或无须提供
• TypeScript 定义已补充或无须补充
• fork仓库代码是否为最新避免文件冲突
• Files changed 没有 package.json lock 等无关文件

Summary by CodeRabbit

• 样式

  • 表单项标签的默认字体大小由14px调整为12px，视觉上更为紧凑。
  • 字体大小现可通过变量灵活配置，提升自定义能力。

• 文档

  • 更新了表单组件文档中的标签字体大小默认值说明，保持与实际样式一致。

[coderabbitai]

## Walkthrough

本次更改将表单项标签的字体大小从固定值改为使用 Sass 变量，并将该变量的默认值由基础字号调整为更小的字号。这样字体大小可以通过变量灵活配置，而不是写死在样式中。

## Changes

| 文件/分组                           | 变更摘要                                               |
|-------------------------------------|--------------------------------------------------------|
| src/packages/formitem/formitem.scss | `.nut-form-item-labeltxt` 的字体大小由 `12px` 改为 `$form-item-label-font-size` 变量 |
| src/styles/variables.scss           | `$form-item-label-font-size` 默认值由 `$font-size-base` 改为 `$font-size-s`        |
| src/packages/form/doc*.md           | 文档中表单项标签字体大小 CSS 变量默认值由 `14px` 改为 `12px`                        |

## Possibly related issues

- jdf2e/nutui-react#3260：该 PR 将表单项标签字体大小从固定值改为可配置变量，直接解决了该 issue 中无法通过 ConfigProvider 修改字体大小的问题。

## Possibly related PRs

- jdf2e/nutui-react#2931：两者都修改了 `.nut-form-item-labeltxt` 样式，虽关注点不同，但都涉及该元素的样式调整。
- jdf2e/nutui-react#2872：两者都将 `.nut-form-item-labeltxt` 的字体大小由固定值改为变量 `$form-item-label-font-size`，代码层面高度相关。
- jdf2e/nutui-react#2927：两者均涉及表单项标签字体样式的调整，且修改了相关 CSS 规则，存在直接关联。

## Suggested reviewers

- oasis-cloud

## Poem

> 兔子挥笔改样式，  
> 字体大小不再死，  
> 变量灵动可配置，  
> 标签更显小巧致。  
> 代码如诗随风舞，  
> UI焕新乐无数！  
> 🐇✨

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📒 Files selected for processing (4)

• src/packages/form/doc.en-US.md (1 hunks)
• src/packages/form/doc.md (1 hunks)
• src/packages/form/doc.taro.md (1 hunks)
• src/packages/form/doc.zh-TW.md (1 hunks)

✅ Files skipped from review due to trivial changes (4)

• src/packages/form/doc.md
• src/packages/form/doc.en-US.md
• src/packages/form/doc.zh-TW.md
• src/packages/form/doc.taro.md

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

src/packages/formitem/formitem.scss (1)

31-34: font-size 重复声明，可适度精简

.nut-form-item-labeltxt 与父选择器 .nut-form-item-label 已同样设置 font-size: $form-item-label-font-size;，且值一致。若无额外层级覆盖需求，可去掉其一以减小 CSS 体积、提高可读性。

📜 Review details

Configuration used: CodeRabbit UI
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📥 Commits

Reviewing files that changed from the base of the PR and between f6f1e06 and 920ab45.

📒 Files selected for processing (2)

• src/packages/formitem/formitem.scss (1 hunks)
• src/styles/variables.scss (1 hunks)

⏰ Context from checks skipped due to timeout of 90000ms (4)

• GitHub Check: build
• GitHub Check: test
• GitHub Check: test
• GitHub Check: lint

🔇 Additional comments (1)

src/styles/variables.scss (1)

2134-2137: 变量默认值改为 $font-size-s 👍

将 $form-item-label-font-size 的默认值从基础字号下调到 $font-size-s（12 px）后，标签字号保持可配置同时更加精细，与设计稿一致，改动无功能性风险。

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.79%. Comparing base (f6f1e06) to head (c9a959e).
Report is 1 commits behind head on feat_v3.x.

Additional details and impacted files

@@            Coverage Diff             @@
##           feat_v3.x    #3280   +/-   ##
==========================================
  Coverage      87.79%   87.79%           
==========================================
  Files            290      290           
  Lines          19089    19089           
  Branches        2939     2939           
==========================================
  Hits           16760    16760           
  Misses          2324     2324           
  Partials           5        5           

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