perf(gravatar): support both sha256 and md5 hash algorithm for gravatar

[qwqcode]

This PR includes the following changes:

• Replaces the default gravatar mirror for zh-CN configuration with WeAvatar
  • The reason is that Cravatar currently does not support the more secure SHA256 encryption
• Supports both md5 and sha256 encryption algorithms, with md5 as the default (for compatibility with older version users).

To enable SHA256 encryption, add sha256=1 to frontend.gravatar.params.

# UI Settings
frontend:
  # Gravatar
  gravatar:
    # API parameters
-   params: d=mp&s=240
+   params: sha256=1&d=mp&s=240

---

[Chinese]

此 PR 包括以下更改：

• 将 zh-CN 配置中的默认 Gravatar 镜像替换为 WeAvatar
  • 原因是 Cravatar 当前不支持更安全的 SHA256 加密
• 同时支持 md5 和 sha256 加密算法，默认使用 md5（为兼容旧版用户）

要启用 SHA256 加密，请在 frontend.gravatar.params 中添加 sha256=1。

fixes #874

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 31.69%. Comparing base (c3558b7) to head (70cece7).

Files	Patch %	Lines
internal/core/base.go	66.66%	2 Missing and 1 partial ⚠️
internal/dao/cook.go	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #912      +/-   ##
==========================================
+ Coverage   31.56%   31.69%   +0.12%     
==========================================
  Files         203      204       +1     
  Lines        8654     8668      +14     
==========================================
+ Hits         2732     2747      +15     
  Misses       5771     5771              
+ Partials      151      150       -1     

Flag	Coverage Δ
go	31.69% <71.42%> (+0.12%)	⬆️

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