fix(wallpapers-ocs): set owner_id to the registered provider id

[perlowja]

WallpaperOcs.items() sets owner_id to the per-network name (for example pling), but the registered provider id is ocs. import_card() calls provider_registry.lookup(item.owner_id), so every OCS import failed with "Wallpaper provider is not active".

Change

One line in wallpapers-ocs/wallpapers_ocs.vala: set each item's owner_id to the provider id after items(). Split out of #4, which keeps only the choices() fix.

Verification

• The failure was confirmed on a live desktop session on 2026-09-21 (import from an OCS collection failed with the message above).
• Commit 02b296f on current main (d255d47) with libsingularity b454931, shell 3b72f26 and desktop master 5c75e56, Debian forky arm64 container: the full tree was configured and built (784 targets) and this commit rebuilt on top (ninja OK);
  ERROR: No such build data file as '/Users/jasonperlow/Projects/ncz-screensavers-pkg/meson-private/build.dat'. 24/24 pass.

Not verified

• The import was not re-run against the live OCS service on this exact commit; the container run covers build and tests only.

AI assistance: disclosed