Replace phone verification with canvas signature on confirm page

Adds a canvas-based SignaturePad component used on the customer
confirm page. Authorize now requires a non-empty signature; reject
opens a required reason modal. The archived signature is shown to
the customer after confirming and on the admin detail page.

Also fixes the confirm page being clipped at the top when its
content exceeds the viewport: the public layout used
height:100vh + overflow:hidden which cropped the centered card.
Switched to min-height:100vh so tall content can scroll naturally.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -92,7 +92,7 @@ VITE_API_BASE_URL=/api
 - 用户登录
 - 公开查询序列号（支持二维码扫描）
   - 扫描到 `zjbf-sh-*` 售后码时自动跳转到售后确认页
-- 售后工单确认页（扫码 → 手机号末四位校验 → 已授权/未授权）
+- 售后工单确认页（扫码 → 签名画板 → 已授权；或填写退回原因 → 未授权）
 
 ### 管理后台