Fix Space thumbnail preview URLs

[zerox80]

Summary

Fixes #171.

This updates Android thumbnail preview URL generation so files inside Spaces use the Space WebDAV endpoint instead of the legacy /webdav path. Space files now resolve previews through /dav/spaces/{spaceId}/..., matching the backend thumbnail routes for SpacesThumbnail.

Root cause

ThumbnailsRequester always built file preview URLs as {baseUrl}/webdav{remotePath}?preview=1. That works for personal files, but images inside Space subfolders need to be requested via the Space WebDAV URL, e.g. /dav/spaces/{spaceId}/{folder}/image.jpg?preview=1.

Backend context: services/webdav/pkg/service/v0/service.go registers both /dav/spaces/{id} and /remote.php/dav/spaces/{id} for Space thumbnail requests.

Changes

• Use OCFileWithSyncInfo.space.root.webDavUrl when available.
• Fall back to {accountBaseUrl}/dav/spaces/{spaceId} for Space files without attached Space metadata.
• Keep personal files on {accountBaseUrl}/webdav.
• Add focused unit coverage for personal URLs, Space-root URLs, Space-ID fallback URLs, and encoded subfolder paths.

Validation

• ./gradlew.bat :opencloudApp:testOriginalDebugUnitTest --tests eu.opencloud.android.presentation.thumbnails.ThumbnailsRequesterTest --no-daemon
• ./gradlew.bat detekt --no-daemon
• ./gradlew.bat assembleDebug --no-daemon