fix: saving settings in theme preview returns to homepage

[LIlGG]

What type of PR is this?

/kind bug
/area ui
/milestone 2.19.x

What this PR does / why we need it:

此 PR 将主题预览及文章预览的 iframe 由 srcdoc 改为使用 src 加载。用于解决在主题预览时，如果点击保存设置，会导致主题端页面跳转至首页。

How to test it?

1. 打开主题预览界面。
2. 打开非首页的页面。
3. 点击保存配置。查看此时刷新页面后是否不再回到首页。

同时需要测试 #4047 中的场景是否还会再发生。

Which issue(s) this PR fixes:

Fixes #6371

Does this PR introduce a user-facing change?

解决预览主题时保存设置，页面会跳转至首页的问题。

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.12%. Comparing base (bc10336) to head (623bc43).
Report is 106 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6447      +/-   ##
============================================
+ Coverage     54.51%   58.12%   +3.61%     
- Complexity     3523     3767     +244     
============================================
  Files           646      651       +5     
  Lines         21862    22106     +244     
  Branches       1528     1555      +27     
============================================
+ Hits          11917    12850     +933     
+ Misses         9328     8633     -695     
- Partials        617      623       +6     

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[guqing]

/lgtm

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ruibaby

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• ui/OWNERS [ruibaby]

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