style(web): favicon 换成登录页 logo 样式(红底白 Z),与 brand 统一

原内联蓝色机器人 SVG 与登录卡/顶栏 brand(红色圆角方块 + 白 Z,
渐变 #c0392b → #8e2a20)不一致;改为同款,浏览器标签页图标对齐品牌。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/static/dev.html

@@ -4,7 +4,7 @@
 <meta charset="utf-8" />
 <title>zcbot 控制台</title>
 <meta name="viewport" content="width=device-width,initial-scale=1" />
-<link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'%3E%3Crect width='100' height='100' rx='20' fill='%232563eb'/%3E%3Crect x='28' y='34' width='44' height='34' rx='8' fill='%23fff'/%3E%3Ccircle cx='40' cy='51' r='5' fill='%232563eb'/%3E%3Ccircle cx='60' cy='51' r='5' fill='%232563eb'/%3E%3Crect x='46' y='18' width='8' height='14' rx='4' fill='%23fff'/%3E%3Ccircle cx='50' cy='16' r='5' fill='%23fff'/%3E%3C/svg%3E" />
+<link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'%3E%3Cdefs%3E%3ClinearGradient id='g' x1='0' y1='0' x2='1' y2='1'%3E%3Cstop offset='0' stop-color='%23c0392b'/%3E%3Cstop offset='1' stop-color='%238e2a20'/%3E%3C/linearGradient%3E%3C/defs%3E%3Crect width='100' height='100' rx='22' fill='url(%23g)'/%3E%3Ctext x='50' y='54' font-family='Arial,Helvetica,sans-serif' font-size='62' font-weight='700' fill='%23fff' text-anchor='middle' dominant-baseline='central'%3EZ%3C/text%3E%3C/svg%3E" />
 
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