feat(wopi): add office document editing

[devld]

Summary

• add discovery-backed WOPI sessions and the CheckFileInfo, GetFile, PutFile, PutRelativeFile, and lock operations needed for document editing
• keep WOPI locks process-local and separate from WebDAV and ordinary drive writes, with best-effort external-change conflict detection
• add an authenticated Office handler that launches the configured editor through a form POST and derives WOPISrc from the current origin for multi-domain deployments
• document the single-setting configuration, reverse-proxy requirements, lock scope, and compatibility boundary in English and Chinese

Compatibility

• WOPI is disabled unless wopi.discovery-url is configured
• the initial supported editor is self-hosted Collabora Online
• sessions and locks are process-local, so WOPI editing currently requires a single go-drive instance
• Microsoft proof-key validation and Microsoft 365 CSPP support are out of scope for this version

Validation

• go test -count=1 ./...
• go test -race -count=1 ./server
• npm run lint
• npm run build-web
• documentation translation, VitePress, and SEO checks
• real Collabora CODE 26.04.2.4 open, lock, save, and unlock flow, including verification of the saved DOCX contents