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.

deploy/push_desktop.sh

@@ -1,11 +1,12 @@
 #!/bin/bash
-# Build the J621 desktop packages and publish them to the update feed.
+# Build the J621 desktop packages, optionally publish them to the website
+# feed.
 #
-# Locally the feed is deploy/data/desktop, which the frontend nginx mounts
-# read-only and serves at /desktop/. With --host the same directory is also
-# copied to a remote deploy checkout (rsync, or tar over ssh when the server
-# has no rsync). electron-updater reads latest-linux.yml / latest.yml from
-# there; the feed URL comes from desktop/electron-builder.yml.
+# Desktop updates no longer depend on this: the app resolves the newest
+# `desktop-v*` release on Gitea at check time (see desktop/README.md). This
+# script builds the packages and can copy them to deploy/data/desktop, which
+# the frontend nginx mounts read-only and serves at /desktop/ for manual
+# downloads and for pre-0.1.2 installs.
 #
 # Usage: ./push_desktop.sh [--win] [--no-build] [--local] [--host user@server:/path]
 #   --win       also cross-build the Windows NSIS installer (needs wine)