fix(api): refresh image export cache when larger file is available

[DestinyleSnowy]

Fixes #949

What changed

Update HTTP API image export cache logic so a later larger image can overwrite an existing smaller cached image.

Why

Previously, if the file already existed, WeFlow skipped writing it, so an exported thumbnail could keep blocking the later high-resolution image.

Testing

• Ran git diff --check

[xunchahaha]

在 electron/services/httpService.ts:1546 这一行：
this.writeFileIfLarger(fullPath, imageBuffer)

这里使用了 imageBuffer，但在这个代码分支中，我们只有 imagePath 变量（原代码是 fs.copyFileSync(imagePath, fullPath)）。这会导致运行时 ReferenceError。

[DestinyleSnowy]

@xunchahaha 如图，可以发现展开代码后，此处调用 writeFileIfLarger 之前已经定义了 imageBuffer：
const imageBuffer = fs.readFileSync(imagePath)

所以这里不会出现 ReferenceError。应该是 GitHub 折叠 diff 时只显示了调用行的问题。