完成产品审查剩余修复：新手指引、反馈入口、付费墙预览

P1-4：新增 WelcomeOverlay 新手指引
- 首次访问时显示 3 步引导：①从地图选城市 ②查看城市简报 ③解锁 Pro 深入分析
- 圆点进度指示、跳过/下一步按钮、点背景可关闭
- 看过一次后 localStorage 标记不再显示

P1-5：付费墙增加功能预览
- HistoryModal 在付费墙上方展示功能说明文案
- 新增 i18n 键 history.previewTitle / history.previewDesc（中英双语）

P2-9：新增反馈入口
- 顶栏增加 Telegram 反馈按钮（MessageCircle 图标）
- 点击跳转 PolyWeather 社群

Tested: npx tsc --noEmit

frontend/components/dashboard/ScanTerminalState.module.css

@@ -314,3 +314,114 @@
     transform: scaleY(1);
   }
 }
+
+/* ── 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); }
+}