fix(webUI): handle non-ASCII filenames in snapshot diff

[JASON-QWeb]

Summary

Fixes https://github.com/anomalyco/opencode/issues/7670 Same as https://github.com/anomalyco/opencode/issues/7408

This PR resolves on Web: when containing non-ASCII filenames, Path is error, so filenames and git diff will display uncorrect and cause SyntaxError.

Problem

• For non-ASCII filenames (CN/KOR/JP) , Git emits quoted paths, so the filenames in session diffs don’t match the ones in snapshot diffs.
• shell interpretation can interfere with git show when paths contain Unicode characters, resulting in empty before/after content when get summary diff.

Changes

• use Bun.spawn to get git show, so it can avoid Bun shell interpretation error
• add decodeGitQuotepath to handle non-ASCII filenames (CN/KOR/JP)

Solution

Make non-ASCII filenames (CN/KOR/JP) path can be decode correctly like other filenames on Web.

Verification

Make changes to three file: test.py , 测试.py and 시험.py

Before:

• Filename is wrong and click will cause error

After: