feat: 样式统一 + 月份下拉框扩展

- 品牌色统一为天机阁标准 #4A6CF7，tailwind.config 扩展完整色阶
- styles.css 新增 :root CSS 变量层（brand 色阶、语义色、圆角/阴影 token）
- 全项目 hex/Tailwind 类统一替换为 brand-* CSS 变量（115 处引用）
- border-radius/box-shadow 统一使用 token 变量
- 预留 :root[data-theme="dark"] 暗色主题变量占位
- 所有月份下拉框范围从 3 年/15 月扩展至 2020-2027（8 年 96 月）

static/modules/drawer.js

@@ -70,7 +70,7 @@ function openDrawer(resource, id) {
     ${resource === "proposals" ? `<section><h3 class="drawer-section-title">附件</h3>${fileGroup("proposal", item.id, "", "附件", item.files || [])}</section>` : ""}
     ${followupTarget ? `<section>
       <h3 class="drawer-section-title">活动 / 跟进</h3>
-      <div class="grid gap-2">${(item.followups || []).map((f) => `<div class="activity-item"><div class="activity-icon"><i data-lucide="message-square"></i></div><div class="min-w-0 flex-1"><div class="flex justify-between gap-3 text-[12px] text-slate-500"><span>${esc(f.follower)} · ${esc(f.follow_up_method)}</span><span>${esc(f.followed_at)}</span></div><div class="mt-1 leading-5 text-slate-800 rich-content" data-html="${encodeURIComponent(f.content || '')}"></div>${f.next_action ? `<p class="mt-1 leading-5 text-blue-700">下一步：${text(f.next_action)}</p>` : ""}</div><button class="activity-delete" onclick="deleteFollowup(event, ${f.id}, '${resource}', ${item.id})" title="删除评论"><i data-lucide="trash-2"></i></button></div>`).join("")}</div>
+      <div class="grid gap-2">${(item.followups || []).map((f) => `<div class="activity-item"><div class="activity-icon"><i data-lucide="message-square"></i></div><div class="min-w-0 flex-1"><div class="flex justify-between gap-3 text-[12px] text-slate-500"><span>${esc(f.follower)} · ${esc(f.follow_up_method)}</span><span>${esc(f.followed_at)}</span></div><div class="mt-1 leading-5 text-slate-800 rich-content" data-html="${encodeURIComponent(f.content || '')}"></div>${f.next_action ? `<p class="mt-1 leading-5 text-brand-700">下一步：${text(f.next_action)}</p>` : ""}</div><button class="activity-delete" onclick="deleteFollowup(event, ${f.id}, '${resource}', ${item.id})" title="删除评论"><i data-lucide="trash-2"></i></button></div>`).join("")}</div>
       <form class="comment-box mt-3" onsubmit="submitComment(event,'${followupTarget}',${item.id},'${resource}')">
         <div class="squire-toolbar">
           <button type="button" class="squire-btn" onmousedown="event.preventDefault();squireCmd('bold')" title="加粗"><i data-lucide="bold"></i></button>