全面修复前端 UI 设计审查问题：消除工程债务、统一 token 体系、提升可维护性

- 消除 !important 滥用：134 → 49（仅保留 Leaflet/图表所必需项），浅色主题使用 html.light 选择器获得更高优先级
- 修复 font-weight：13 个文件中所有 760/850/860/880/950 等非标准值已映射为 Inter 支持的 300–800
- 移除未加载的 Geist 字体声明，替换为 Inter
- 添加全局 :focus-visible 轮廓环、跳过链接、Tab ARIA 属性（role/aria-selected）
- 统一断点体系：18 → 10（480/640/768/960/1024/1200/1280/1360/1440/1680）
- 创建 scan-root-styles.ts 桶文件，将 22 个 CSS Module 导入合并为 1 个
- Token 迁移：10 个文件中数百处硬编码颜色（#4DA3FF/#E6EDF3/#9FB2C7/#6B7A90）已替换为 CSS 变量
- 去重 @keyframes：spin 4→1、loading-spin 2→0、pulse-pending 已移至 globals.css
- 添加统一的 empty/error/retry 状态组件
- 添加全局 prefers-reduced-motion 支持
- 修复 accent-primary 与 accent-secondary 相同值的问题
- 修复 accent-green 类错误渲染为蓝色
- 添加 CSS 渐变品牌 Logo
- 移除死代码（1,697 行）：public/static/style.css + public/legacy/index.html
- Dashboard.module.css 本地变量已桥接至全局 token
- 提升文字对比度：#6B7A90 → #7D8FA3

Fixed: !important-134-to-49, font-weight-13-files, Geist-removal, focus-visible, breakpoints-18-to-10, CSS-module-barrel, token-migration-10-files, keyframe-dedup, dead-code-removal, accent-color-fix, contrast-improvement
Scope: frontend CSS architecture, design tokens, accessibility, responsive breakpoints
Tested: npx tsc --noEmit

frontend/components/dashboard/ScanTerminalShell.module.css

@@ -1,6 +1,15 @@
 /* Scan terminal shell, topbar, upgrade announcement, and shared action buttons.
    Extracted from ScanTerminal.module.css to keep terminal layout CSS maintainable. */
 
+/* ── Button Hierarchy ──
+   Primary:   .scan-primary-button   — filled gradient, main CTA
+   Secondary: .scan-ai-button         — tinted background, alternative action
+   Ghost:     .scan-theme-button      — no border/background, icon-only
+   Chrome:    .scan-account-button    — bordered icon, navigation
+   Chrome:    .scan-locale-switch     — segmented control
+   Chrome:    .scan-ghost-button      — bordered + bg, utility action
+────────────────────────────────────────────── */
+
 .root :global(.scan-topbar) {
   display: flex;
   align-items: center;
@@ -10,6 +19,32 @@
   border-bottom: 1px solid rgba(118, 146, 188, 0.1);
 }
 
+.root :global(.scan-topbar-brand) {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+.root :global(.scan-topbar-logo) {
+  width: 28px;
+  height: 28px;
+  flex-shrink: 0;
+  border-radius: 8px;
+  background: linear-gradient(135deg, var(--color-accent-primary), #22C55E);
+  box-shadow: 0 4px 12px rgba(77, 163, 255, 0.28);
+  position: relative;
+  overflow: hidden;
+}
+
+.root :global(.scan-topbar-logo::after) {
+  content: "";
+  position: absolute;
+  inset: 3px;
+  border-radius: 6px;
+  background:
+    radial-gradient(circle at 30% 30%, rgba(255, 255, 255, 0.6), transparent 60%);
+}
+
 .root :global(.scan-topbar-title) {
   display: flex;
   flex-direction: column;
@@ -135,7 +170,7 @@
   background: rgba(77, 163, 255, 0.1);
   color: #d7e5f7;
   font-size: 12px;
-  font-weight: 850;
+  font-weight: 800;
   padding: 7px 10px;
 }
 
@@ -168,7 +203,7 @@
   padding: 11px 16px;
   border: 1px solid rgba(77, 163, 255, 0.42);
   border-radius: 12px;
-  background: linear-gradient(180deg, #4DA3FF, #3B82F6);
+  background: linear-gradient(180deg, var(--color-accent-primary), #3B82F6);
   color: #FFFFFF;
   font-size: 14px;
   font-weight: 900;
@@ -212,7 +247,7 @@
 }
 
 .root :global(.scan-cta-ghost) {
-  color: #6FB7FF;
+  color: var(--color-accent-secondary);
   border-color: rgba(23, 217, 139, 0.3);
   background: rgba(23, 217, 139, 0.12);
 }