docs: rebuild README previews from 2x captures

Rework the product preview section around three real screens captured at
deviceScaleFactor 2 instead of the previous 900 px shots taken at a 75%
browser zoom, which read as blurry.

- capture the conversation workspace, the agent customization page and the
  visitor widget, then crop each one to a consistent frame
- drop the tight 480 px widget crop and its leftover gray strip
- restructure the preview section with one lead line and a caption per shot,
  and fold the local start-up steps into a shorter quick start
- keep the browser overlay out of the captures and note the method in the
  asset manifest
- finish the visible ReplyPier rename (sidebar wordmark, widget footer,
  page title, login screen, demo page) so screenshots and UI agree

frontend/src/components/layout/AppSidebar.vue

@@ -67,7 +67,7 @@ const initial = computed(() => (props.userName || '·').trim().slice(0, 1))
         <div class="brand">
             <BrandMark :size="34" />
             <div v-if="!isCollapsed" class="brand-text">
-                <span class="wordmark">Chatter<i>Mate</i></span>
+                <span class="wordmark">Reply<i>Pier</i></span>
                 <span class="brand-sub">智能客服工作台</span>
             </div>
             <SidebarToggle :isCollapsed="isCollapsed" @toggle="emit('toggle')" />

frontend/src/views/LoginView.vue

@@ -287,7 +287,7 @@ const handleVerifyAndResetPassword = async () => {
         <section class="form-col">
             <div class="form-top reveal" style="--i: 0">
                 <BrandMark :size="34" />
-                <span class="wordmark" style="font-size: 21px">Chatter<i>Mate</i></span>
+                <span class="wordmark" style="font-size: 21px">Reply<i>Pier</i></span>
                 <button
                     class="icon-btn"
                     @click="toggleTheme"