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/about/founded-history.vue

@@ -131,7 +131,7 @@
                   :class="[
                     'px-4',
                     'py-2',
-                    idx < 2 ? 'font-bold text-primary' : '',
+                    idx < 2 ? 'font-bold text-primary-400' : '',
                   ]"
                 >
                   {{ donor.amount }}
@@ -167,7 +167,7 @@
                   {{ money.format(serviceDonor.amount) }}
                 </td>
               </tr>
-              <tr class="bg-amber-50 font-semibold text-primary">
+              <tr class="bg-amber-50 font-semibold text-primary-400">
                 <td class="px-4 py-2">总额</td>
                 <td class="px-4 py-2">专业服务及装修项目</td>
                 <td class="px-4 py-2">{{ money.format(33170) }}</td>

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"

app/pages/join-us/index.vue

@@ -113,7 +113,7 @@ const handleSubmit = () => {
 </script>
 
 <template>
-  <UPage class="bg-primary">
+  <UPage class="bg-primary-400">
     <div
       class="cursor-not-allowed fixed flex items-center justify-center min-h-screen min-w-screen bg-black opacity-50"
     >