fix(drivers/pikpak): strip subdomain prefix from OSS endpoint for all platforms

[ericwang2006]

…ms #2431

Summary

PikPak API returns upload endpoints with a server-specific subdomain prefix
(e.g. vip-lixian-07.upload-a10b.mypikpak.com). These per-node hostnames are
often unresolvable via DNS. The previous code only normalized the endpoint for
the android platform; web and pc platforms used the raw API value,
causing uploads to fail with no such host.

Change: Removed the platform guard so the subdomain prefix is stripped for
all platforms, keeping only the stable base OSS domain.

• This PR has breaking changes.
• This PR changes public API, config, storage format, or migration behavior.
• This PR requires corresponding changes in related repositories.

Testing

• go test ./... (not run: go.mod requires Go 1.25+, local toolchain too old)
• Manual test: upload a file to PikPak storage configured with web or pc platform

Checklist

• I have read CONTRIBUTING.
• I confirm this contribution follows the repository license, contribution policy, and code of conduct.
• I have formatted the changed code with gofmt.
• I have requested review from relevant maintainers or code owners where applicable.

AI Disclosure

• This PR includes AI-assisted content.

Tools used:

• OpenCode

Usage scope:

• Code generation

• Review assistance

• I have reviewed and validated all AI-assisted content included in this PR.

• I can reproduce all AI-assisted content included in this PR without any AI tools.