清理死代码：移除 14 个孤立组件/模块，Koyfin 品牌改为 PolyWeather

删除的文件：
- ContinentGroupHeader, AiPinnedForecastView, DataFreshnessBar, MarketDecisionLine
- opportunity-ai-meta/evidence-summary/v4-decision/v4-risk
- HeaderBar, IntradaySignalScene, MapCanvas, PolyWeatherDashboard
- ScanFilterPanel, WeatherAuraLayer

修改：
- decisionClass 死函数移除
- KoyfinWeatherTerminal → PolyWeatherTerminal
- Koyfin-style grid → Multi-panel grid
- 欢迎覆盖层 CSS 清理

frontend/components/dashboard/ScanTerminalState.module.css

@@ -181,113 +181,3 @@
   }
 }
 
-/* ── Welcome onboarding overlay ── */
-.root :global(.scan-welcome-overlay) {
-  position: fixed;
-  inset: 0;
-  z-index: 1000;
-  display: grid;
-  place-items: center;
-  background: rgba(2, 6, 23, 0.72);
-  backdrop-filter: blur(4px);
-  animation: fadeIn 0.25s ease;
-  padding: 24px;
-}
-
-.root :global(.scan-welcome-card) {
-  width: 100%;
-  max-width: 400px;
-  padding: 32px 28px 24px;
-  border: 1px solid rgba(77, 163, 255, 0.22);
-  border-radius: 24px;
-  background: linear-gradient(160deg, rgba(17, 26, 46, 0.98), rgba(11, 18, 32, 0.98));
-  box-shadow: 0 24px 80px rgba(0, 0, 0, 0.5);
-  text-align: center;
-  animation: fadeUpIn 0.3s ease;
-}
-
-.root :global(.scan-welcome-steps) {
-  display: flex;
-  justify-content: center;
-  gap: 8px;
-  margin-bottom: 20px;
-}
-
-.root :global(.scan-welcome-dot) {
-  width: 8px;
-  height: 8px;
-  border-radius: 999px;
-  background: rgba(77, 163, 255, 0.2);
-  transition: all 0.2s;
-}
-
-.root :global(.scan-welcome-dot.active) {
-  width: 24px;
-  background: var(--color-accent-primary);
-}
-
-.root :global(.scan-welcome-dot.done) {
-  background: rgba(77, 163, 255, 0.5);
-}
-
-.root :global(.scan-welcome-icon-wrap) {
-  width: 64px;
-  height: 64px;
-  margin: 0 auto 16px;
-  display: grid;
-  place-items: center;
-  border-radius: 18px;
-  background: rgba(77, 163, 255, 0.12);
-  border: 1px solid rgba(77, 163, 255, 0.18);
-}
-
-.root :global(.scan-welcome-icon) {
-  color: var(--color-accent-secondary);
-}
-
-.root :global(.scan-welcome-title) {
-  font-size: 20px;
-  font-weight: 800;
-  color: var(--color-text-primary);
-  margin: 0 0 8px;
-}
-
-.root :global(.scan-welcome-desc) {
-  font-size: 14px;
-  color: var(--color-text-secondary);
-  line-height: 1.6;
-  margin: 0 0 24px;
-}
-
-.root :global(.scan-welcome-actions) {
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  gap: 12px;
-}
-
-.root :global(.scan-welcome-skip) {
-  border: none;
-  background: transparent;
-  color: var(--color-text-muted);
-  font-size: 13px;
-  font-weight: 600;
-  cursor: pointer;
-  padding: 8px 12px;
-  border-radius: 8px;
-  transition: color 0.15s;
-}
-
-.root :global(.scan-welcome-skip:hover) {
-  color: var(--color-text-secondary);
-}
-
-@keyframes fadeIn {
-  from { opacity: 0; }
-  to { opacity: 1; }
-}
-
-@keyframes fadeUpIn {
-  from { opacity: 0; transform: translateY(12px); }
-  to { opacity: 1; transform: translateY(0); }
-}