refactor: unified brand naming to BoosChat

Rebrand all user-facing product name references from legacy
Chinese name to unified "BoosChat" across frontend components,
metadata, test fixtures, and documentation.

web/components/app-sidebar.tsx

@@ -49,12 +49,12 @@ export function AppSidebar({ ...props }: ComponentProps<typeof Sidebar>) {
             >
               <img
                 src="/images/logo.svg"
-                alt="贝壳AGENT"
+                alt="BoosChat"
                 width="32"
                 height="32"
                 className="size-7 shrink-0 object-contain"
               />
-              <span className="text-base font-semibold">贝壳AGENT</span>
+              <span className="text-base font-semibold">BoosChat</span>
             </SidebarMenuButton>
           </SidebarMenuItem>
         </SidebarMenu>

web/components/login-form.tsx

@@ -84,9 +84,9 @@ export function LoginForm({
       <FieldGroup>
         <div className="flex flex-col gap-2 text-center">
           <span className="mx-auto inline-flex rounded-full border border-amber-300/60 bg-amber-50 px-3 py-1 text-[11px] font-medium tracking-[0.22em] text-amber-900 uppercase">
-            贝壳AI
+            BoosChat
           </span>
-          <h1 className="text-3xl font-semibold tracking-tight">欢迎使用贝壳 AI 客服平台</h1>
+          <h1 className="text-3xl font-semibold tracking-tight">欢迎使用 BoosChat 客服平台</h1>
         </div>
         <Field>
           <FieldLabel htmlFor="username">用户名</FieldLabel>