fix: avoid permanent start-edge fade on browsers without scroll-drive…

[xmengnet]

Summary

shadcn/tailwind.css 提供的 scroll-fade-x/y 工具类依赖 CSS scroll-driven animations（animation-timeline: scroll()）实现"滚动后才显示边缘渐隐"，但在不支持的浏览器（Firefox < 154、Safari < 26、Chromium < 115）会落入 @supports not 静态回退分支：左右/上下边缘渐隐永久常驻，与滚动位置无关。

由于侧栏会话标题的遮罩元素与文字零间距（32px 缩进加在父级 Link 上），导致每个标题的首字永久被淡化约一半（见截图）；对话中的 Markdown 表格在未滚动时左列也被淡化。Chrome 115+ 走滚动驱动分支表现正常，因此该问题此前未被发现。

修复方式：在 globals.css 末尾追加一段顶层 @supports not (animation-timeline: scroll()) 覆盖，仅在不支持滚动驱动动画的浏览器中将起始侧渐隐变量置 0（--scroll-fade-s / --scroll-fade-t），保留末端"还有更多内容"的提示。使用通配选择器（unlayered 规则优先于 @layer utilities），可同时覆盖 sm:scroll-fade-x 等变体类。

影响范围（一次性修复全部 6 处使用点）：侧栏会话标题（项目/最近/星标共用）、对话 Markdown 表格、聊天输入框附件列表、附件条、回复大纲/消息定位 rail、导航搜索。

支持滚动驱动动画的浏览器完全不受影响；属渐进增强，Firefox 154+ 将自动获得完整动态效果。

Change type

• Bug fix

Affected areas

• Frontend / UI

Verification

• pnpm --filter @deeix/web build 构建通过（含 next build 类型检查）
• 用项目 Tailwind v4 PostCSS 管道编译 globals.css，确认覆盖规则位于顶层、unlayered 优先于 utilities 层，且产物包含 @supports not (animation-timeline:scroll()){*{--scroll-fade-s:0px;--scroll-fade-t:0px}}
• 手动浏览器验证：Firefox 修复前侧栏标题首字/表格左列被淡化，修复后显示完整；Chrome 表现与修改前一致

Screenshots, API examples, or logs

修复前，左侧一直显示不全：

修复后：

Configuration, migration, and compatibility notes

无配置、环境变量、数据库、API 契约变更。纯 CSS 渐进增强，向后兼容；旧浏览器仅失去滚动过程中的起始侧装饰性渐隐过渡，末端提示保留。

Documentation

• Documentation is not needed for this change.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.