refactor(web): 提取共享的圆形多选勾选框组件，统一5处重复实现

创建群聊、消息多选、我的收藏多选、黑名单多选、转发选择器——这5个
地方各自手写了一份几乎一样的"圆形+选中变实心+里面check图标"勾选框
markup，样式上有细微且无意的差异（比如有的选中态多画了一圈边框、
用的灰色深浅不完全一致），上次只修了转发选择器一处，用户指出创建
群聊那里同样丑、而且问了"这种组件不该统一做吗"——確实该統一。

新增 `app/SelectionCheckbox.tsx`，把上次在转发选择器里验证过的
矢量勾选图标方案抽成共享组件，5处全部换成调用它。以后这个视觉样式
要再调整，改一个文件就够了，不会再出现"修好了一处，另外四处还是
老样子"的情况。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/features/messages/ForwardPickerModal.tsx

@@ -3,6 +3,7 @@ import { useMemo, useState } from 'react';
 import type { Contact } from 'tinode-sdk';
 
 import { Icon } from '@/app/Icon';
+import { SelectionCheckbox } from '@/app/SelectionCheckbox';
 import { avatarBgClass, avatarLetter } from '@/lib/avatar';
 import { useEscapeKey } from '@/lib/hooks';
 import { useChatList, useTinode } from '@/lib/tinode/hooks';
@@ -14,19 +15,6 @@ interface ForwardPickerModalProps {
   onClose: () => void;
 }
 
-/** A hand-drawn checkmark instead of the Material Icons font glyph this row used to use -- the font
- *  glyph's hinting isn't tuned for a 14px render inside a 20px circle, so at anything above 1x pixel
- *  density it comes out visibly soft/asymmetric where a vector path stays crisp at any scale. Not a
- *  systemic swap (`Icon` is still how the rest of the app draws icons) -- scoped to this one
- *  selection indicator, which is the one a user actually screenshotted and flagged as rough-looking. */
-function CheckMark({ className = '' }: { className?: string }) {
-  return (
-    <svg viewBox="0 0 16 16" fill="none" className={className} aria-hidden="true">
-      <path d="M3.5 8.5L6.5 11.5L12.5 4.5" stroke="currentColor" strokeWidth="1.75" strokeLinecap="round" strokeLinejoin="round" />
-    </svg>
-  );
-}
-
 /**
  * README §3.6: multi-select targets + an optional comment, sent in one action. Used by the message
  * context menu's single-message "转发", the multi-select bulk bar's "转发", and favorites' bulk
@@ -115,11 +103,7 @@ export function ForwardPickerModal({ onPick, onClose }: ForwardPickerModalProps)
           {avatarLetter(c.public?.fn)}
         </span>
         <span className="min-w-0 flex-1 truncate text-sm text-text-body">{c.public?.fn || c.topic}</span>
-        <span
-          className={`grid h-5 w-5 flex-none place-items-center rounded-full ${isSelected ? 'bg-accent' : 'border-[1.5px] border-[#c5cad2]'}`}
-        >
-          {isSelected ? <CheckMark className="h-3 w-3 text-white" /> : null}
-        </span>
+        <SelectionCheckbox selected={isSelected} />
       </button>
     );
   };