style(theme): expand color palettes and refine dark mode styles

- Expand CSS variables for primary and secondary colors to include full 50-950 scales in `main.css`.
- Update components to reference specific color shades (e.g., `primary-400`, `secondary-200`) instead of generic variables.
- Add dark mode background and text colors to Events, Hall of Fame, and Index sections.
- Adjust image aspect ratio in the Events component.

app/pages/index.vue

@@ -19,7 +19,7 @@
 
     <!-- 捐赠模块 -->
     <UPageCTA
-      class="bg-secondary"
+      class="bg-secondary-200 dark:bg-secondary-900"
       title="支持与捐赠（功能暂未开放）"
       description="您的捐赠将用于奖学金、校园建设及校友活动发展。感谢您对母校的支持！"
       :links="donationLinks"