🐛 Fix(gui): create the clipboard image folder before built-in clipboard uploads - #1460

With "use built-in clipboard" turned on, uploading a screenshot from the clipboard does nothing when <config dir>/picgo-clipboard-images doesn't exist. uploadWithBuildInClipboard writes the PNG there with write-file-atomic, which doesn't create parent folders, so the write throws ENOENT, the error only goes to the log and the upload never starts.

PicGo-Core creates that folder only when its own clipboard script runs, so the folder can be missing on a fresh config dir, or after switching to a custom config path, if the built-in clipboard is the only one used there.

This is the error in #1375 (Windows; the reporter found that turning the built-in clipboard off made it work), #1364 and #1263 (macOS). The .png.<number> suffix in those logs is write-file-atomic's temp file, so they come from this code path. The toolbox fix works around it by creating the folder; this change creates it right before the write.

• fse.ensureDir() on the folder before writing the clipboard image
• test: a built-in clipboard upload into a fresh temp config dir (real fs) now uploads instead of returning false

pnpm vitest run src/__tests__/main/uploader-context.spec.ts fails before the change and passes after; the full pnpm test suite, tsc --noEmit and eslint on the touched files pass.