Point desktop updates at the Gitea release feed

The updater now resolves the newest non-draft desktop-v* release through the
Gitea API at check time (J621_UPDATE_REPO, lowercase because the API path is
case-sensitive), picks the platform's latest*.yml asset and uses that release
as a generic electron-updater feed; J621_UPDATE_URL still overrides
everything. Verified against the live API: release picked, yml fetched,
artifact HEAD 200.

electron-builder's publish.url is now metadata only (still needed so the
build emits latest*.yml). Docs updated: CD release assets are the feed, the
website /desktop/ feed only matters for installs before 0.1.2.

AGENTS.md

@@ -56,9 +56,10 @@ Project constraints (do not regress):
   `desktop-v<version>`). Jobs run on the user-scoped runners: `ubuntu-latest`
   on nitro-ci, `desktop` on msi-mortar-ci. Do not add actions/cache
   (`cache: pip`/`npm`) to these workflows: Gitea's cache service hangs the job
-  on restore/save. The live desktop update feed (deploy/data/desktop) is still
-  published with `deploy/push_desktop.sh --no-build` from a machine with SSH
-  to the deploy host — CI has no key for that.
+  on restore/save. Desktop updates read the release assets (latest*.yml) from
+  the Gitea API at check time; the older website feed (deploy/data/desktop)
+  only matters for installs before 0.1.2 and is refreshed with
+  `deploy/push_desktop.sh` from a machine with SSH to the deploy host.
 - Security/permission tests live in backend/apps/core/tests and need a
   one-time grant: GRANT ALL ON `test_j621`.* TO 'j621'@'%';