fix(web): 图标尺寸覆盖(text-[Npx])在任何地方都不生效——CSS层级冲突

根因：Material Icons的Google Fonts样式表(index.html的<link>)自己的
.material-icons规则完全没有@layer包裹（纯普通CSS），而Tailwind v4的
每一个工具类都在@layer utilities里——CSS Cascade Layers规范规定，任
何未分层的声明，不管选择器优先级多高、样式表加载顺序多晚，永远优先于
任何分层声明。实测坐实：<Icon className="text-[14px]">的计算样式恒
定是font-size:24px，不管试图覆盖成几px都没用，因为Google那条未分层
规则永远赢。这不是某一处的问题，是Icon组件全局的、此前从未被发现的
系统性bug——历史上多轮视觉审查用截图核对布局，没人具体去量过某个
图标的computed font-size。

修复：Icon.tsx改用自己的im-icon类名，不再用material-icons这个
Google样式表实际瞄准的类名，这样那条未分层规则从一开始就不会匹配到
这些元素。index.css里用@layer base原样照抄了Material Icons自己那条
规则的视觉默认值(24px/line-height:1等)，套上分层之后，Tailwind自己
的@layer utilities（在Tailwind的层注册顺序里排在base之后）就能正常
覆盖它了——和每一处写`text-[Npx]`时本来就该有的效果一致。

全局仓库搜索确认material-icons这个类名只有Icon.tsx一处引用，改动范围
可控、不涉及其它代码路径。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/app/Icon.tsx

@@ -1,10 +1,15 @@
 /** Thin wrapper around a Material Icons ligature glyph (font loaded in index.html).
  *  Always `aria-hidden` -- the glyph's DOM text is the icon's internal ligature name (e.g.
  *  "arrow_back_ios_new"), never something a screen reader should announce. An icon-only
- *  interactive element (button/link with no visible text) must set its own `aria-label`. */
+ *  interactive element (button/link with no visible text) must set its own `aria-label`.
+ *
+ *  Deliberately `im-icon`, NOT Google Fonts' own `.material-icons` class -- see index.css's own
+ *  comment on that rule for why: Google's stylesheet ships that class completely unlayered, which
+ *  under CSS Cascade Layers always outranks Tailwind's layered utilities regardless of specificity
+ *  or source order, silently ignoring every `text-[Npx]` size override across the whole app. */
 export function Icon({ name, className = '', style }: { name: string; className?: string; style?: React.CSSProperties }) {
   return (
-    <i aria-hidden="true" className={`material-icons select-none ${className}`} style={style}>
+    <i aria-hidden="true" className={`im-icon select-none ${className}`} style={style}>
       {name}
     </i>
   );

web/src/index.css

@@ -1,5 +1,35 @@
 @import "tailwindcss";
 
+/*
+ * Material Icons' OWN Google Fonts stylesheet (index.html's `<link>`) ships its `.material-icons`
+ * class completely unlayered -- plain CSS, no `@layer`. Tailwind v4 (the `@import` above) puts every
+ * one of its own utilities inside `@layer utilities`. Per the CSS Cascade Layers spec, ANY unlayered
+ * declaration beats EVERY layered one for the same property on the same element, regardless of
+ * selector specificity or which stylesheet loads later in the document -- confirmed live: an
+ * `<Icon className="text-[14px]">` computed to `font-size: 24px` regardless of the requested size,
+ * every time, because Google's unlayered rule was always winning over Tailwind's layered
+ * `.text-\[14px\]` utility. Icon.tsx uses its own `im-icon` class (not `.material-icons`) specifically
+ * so Google's competing rule never matches these elements at all -- this is the same visual default
+ * (24px, line-height 1, etc., copied 1:1 from Google's own stylesheet), just declared inside `@layer
+ * base` so Tailwind's `@layer utilities` (which Tailwind itself registers after `base`) can actually
+ * override it, the way every call site sizing an icon via `text-[Npx]` already assumed it could.
+ */
+@layer base {
+  .im-icon {
+    font-family: 'Material Icons';
+    font-weight: normal;
+    font-style: normal;
+    font-size: 24px;
+    line-height: 1;
+    letter-spacing: normal;
+    text-transform: none;
+    display: inline-block;
+    white-space: nowrap;
+    word-wrap: normal;
+    direction: ltr;
+  }
+}
+
 /*
  * Design tokens ported 1:1 from docs/design_handoff_meetingim_web/README.md's "Design Tokens"
  * section. Values are exact hex/px from the design handoff, not semantic guesses -- keep them in