`Url.lastSegment` conflates "empty path" with a literal decoded slash

[rfc2822]

DavUtils.lastSegment (core/src/main/kotlin/at/bitfire/davdroid/util/DavUtils.kt) returns "/" in two distinct cases:

1. the URL has no path segments at all (e.g. https://example.com → "/"), and
2. the URL's last raw path segment is %2F, which decodes to a literal / character.

These are indistinguishable to callers. For display-only call sites (HomeSet.title() etc) this is harmless. But InternalMemberState.fileName (and anything that derives a local storage identity from it, e.g. SyncManager.decideDownload/findByName) relies on the same helper, so a remote member whose name decodes to a literal / would collide with the "no segment" sentinel.

Found while auditing decode/encode consistency for #2755's listChanges extraction (see core/src/test/kotlin/at/bitfire/davdroid/util/DavUtilsTest.kt:58, which pins the current "/"-for-empty-path behavior).

Possible fix:

• Make Url.lastSegment nullable (String?), returning null only for a truly empty path; push the ?: "/" fallback into the display call sites that want it.
• Make sure that the same helpers are always used to extract the file name from an URL, and to append a file name to a collection URL.
• Probably unify with dav4jvm helpers.