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.

desktop/electron-builder.yml

@@ -2,12 +2,13 @@ appId: io.j621.desktop
 productName: J621
 copyright: Copyright (c) 2026 JakeBreath — Jake Labs Non-Commercial Software Licence
 
-# Update feed served by the frontend nginx (deploy/data/desktop, published
-# with deploy/push_desktop.sh). Baked into resources/app-update.yml; override
-# at runtime with J621_UPDATE_URL for a fork or a test feed.
+# Update feed: desktop/src/main.ts resolves the newest desktop-v* release on
+# Gitea at check time (J621_UPDATE_REPO). This block only tells electron-builder
+# to emit latest.yml/latest-linux.yml next to the installers; J621_UPDATE_URL
+# overrides the feed for a fork or a test.
 publish:
   provider: generic
-  url: https://j621.rainbow-herring.ts.net/desktop
+  url: https://gitea.rainbow-herring.ts.net/JakeBreath/J621/releases
 
 directories:
   output: release