ADFA-5004 feat(kiwix): delete installed ZIMs from the app

[luisguzman-adfa]

Restores the ability to remove installed Wikipedia/ZIM content to free space,
which was lost when the legacy web dashboard was retired (ADFA-4933) and never
ported to the REST core.

Backend adds two direct endpoints to the dashboard, mirroring /maps/delete:
GET /kiwix/library lists installed ZIMs, and POST /kiwix/delete removes one
and rebuilds the Kiwix index (iiab-make-kiwix-lib). The delete logic is ported
from the retired delete_zim socket handler and hardened with strict name
validation (a plain <file>.zim, plus a basename guard) so it can't escape the
ZIM directory.

On the app side, a native KiwixClient (mirroring MapsRegionClient /
BooksClient) backs a small manage screen that lists installed ZIMs with a
per-item delete and a confirm dialog — the same in-app pattern as Maps, rather
than injecting controls into kiwix-serve's upstream page.

Add k2go_zim_* (manage/list/delete + deferred) to every values-*/strings_k2go.xml,
matching the base English keys. Placeholders (%1$d/%2$s/%1$s) preserved. Initial
AI translations — pending native-speaker review.

Verified on device: after delete the ZIM is gone from the list and from Read
Wikipedia. No changes to the download job or to kiwix-serve.