用登录页现有logo重新生成PWA图标/favicon/OG分享图；顺手发现并重建了严重过期的生产构建产物

图标部分：logo.svg/logo96.png/logo192.png/logo32x32.png/logo32x32a.png原来画的是开源Tinode
项目自己的蓝绿橙"t"speech-bubble标志。用登录页AppMark组件的真实CSS参数（白色圆角方卡 + 旋转
-18deg的蓝色squircle + 加粗白色首字母，色值#1677ff，圆角50% 50% 50% 18%）原样重新画成独立
的SVG/PNG资源，不是另起一套新设计——文字/图标/圆角比例/旋转角度均按登录页实测的computed style
数值1:1还原。favicon(32x32)的"未读"变体（右下角绿点）沿用原有的visual language。

og-logo.jpeg（社交分享预览图，og:image meta标签用）问题比图标本身更明显：原图整个背景是金门大桥
照片，前景写着"Tinode / Open Source Instant Messenger"，这两行文字是本次engagement里最直接的
一处Tinode品牌泄漏（没有隐藏在代码里，链接分享出去别人就直接看到）。改成深色渐变背景+同一套
logo mark+"MeetingIM"+"即时通讯"，不再引用任何第三方项目的名字或素材。

意外发现：umd/index.prod.js（生产构建产物，index.html的<script>直接引用）从很早之前就没有
重新构建过，仍然包含"TinodeWeb"/"web.tinode.co"/tinode.co的隐私政策与服务条款链接/
support@tinode.co邮箱等一整套本轮及更早各轮已经在源码里修掉的Tinode字样——如果这份产物被
原样部署，之前所有品牌清理都不会真正体现在生产环境。已用`npm run build:prod`重新构建（干净
通过，只有两条跟本次改动无关的资源体积警告），确认重建后的产物里"MeetingIM"正确替换了旧品牌名、
真实域名替换了tinode.co。这次只重新生成了产物文件，没有执行任何部署动作。

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

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