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/app/SelectionCheckbox.tsx

@@ -0,0 +1,32 @@
+interface SelectionCheckboxProps {
+  selected: boolean;
+  className?: string;
+}
+
+/**
+ * Shared circular multi-select indicator -- every list in this app that lets you pick multiple
+ * items (forward targets, message multi-select, create-group contact picker, favorites, blocklist)
+ * used to hand-copy this same circle-that-fills-with-a-checkmark markup into its own file, each
+ * with small, accidental class differences (a stray extra border in the selected state here, a
+ * slightly different gray there) -- fixing the same visual bug five separate times is exactly the
+ * failure mode a shared component exists to prevent.
+ *
+ * Draws the checkmark as a hand-drawn SVG path, not the Material Icons font glyph `Icon` uses
+ * everywhere else: the font's hinting isn't tuned for a ~13px glyph inside a 20px circle, so above
+ * 1x pixel density it renders visibly soft/asymmetric where a vector path stays crisp at any scale.
+ * A deliberate one-off exception to this app's usual `Icon`-for-everything convention, not a second
+ * icon system -- scoped to this one selection indicator.
+ */
+export function SelectionCheckbox({ selected, className = '' }: SelectionCheckboxProps) {
+  return (
+    <span
+      className={`grid h-5 w-5 flex-none place-items-center rounded-full ${selected ? 'bg-accent' : 'border-[1.5px] border-[#c5cad2] bg-white'} ${className}`}
+    >
+      {selected ? (
+        <svg viewBox="0 0 16 16" fill="none" className="h-3 w-3 text-white" 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>
+      ) : null}
+    </span>
+  );
+}