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.

.gitea/workflows/cd.yml

@@ -7,10 +7,12 @@
 #   * builds the desktop packages and attaches them (plus the update
 #     metadata) to the Gitea release tagged `desktop-v<package.json version>`.
 #
-# The live update feed (deploy/data/desktop, served by the frontend nginx at
-# /desktop/) is not touched here: it is runtime state on the deploy host and
-# is still published with `deploy/push_desktop.sh --no-build` from a machine
-# that can reach it.
+# The desktop build also attaches the update metadata (latest*.yml) to the
+# release; that is the desktop updater's feed, resolved through the Gitea API
+# at check time (see desktop/README.md). The older website feed
+# (deploy/data/desktop, served at /desktop/) is runtime state on the deploy
+# host and only needed for installs before 0.1.2; it is refreshed with
+# `deploy/push_desktop.sh` from a machine that can reach the deploy host.
 #
 # Registry login uses a repo PAT with the minimal write:package scope (the
 # Gitea registry rejects the automatic job token, go-gitea/gitea#23642);