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/layouts/DashboardLayout.vue

@@ -372,7 +372,7 @@ const openNotificationsFromSheet = () => {
                  footer would clip the last message / message input out of view. -->
             <footer v-if="!props.hideHeader && !props.flush" class="footer">
                 <div class="footer-content">
-                    <p>&copy; 2024 ChatterMate。保留所有权利。</p>
+                    <p>&copy; 2024 ReplyPier。保留所有权利。</p>
                     <nav class="footer-links">
                         <a href="https://chattermate.chat/privacy_policy.html">隐私政策</a>
                         <a href="https://chattermate.chat/terms_and_conditions.html">服务条款</a>