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/src/main.ts

@@ -238,8 +238,9 @@ function isDownloadNavigation(url: URL): boolean {
 /**
  * Updates are manual by design: Linux packages install through pacman/dpkg
  * (pkexec/sudo) and the Windows build is unsigned, so the app asks before
- * downloading and again before installing. The feed comes from `publish` in
- * electron-builder.yml and can be overridden with J621_UPDATE_URL.
+ * downloading and again before installing. The feed is resolved at check time
+ * from the newest `desktop-v*` release on Gitea (`J621_UPDATE_REPO`);
+ * `J621_UPDATE_URL` overrides it for forks and the smoke test.
  */
 type UpdateEvent =
   | { state: "available"; version: string }
@@ -250,9 +251,84 @@ type UpdateEvent =
 let updateReporter: ((event: UpdateEvent) => void) | null = null;
 let updateCheckRunning = false;
 
-function setUpUpdates(win: BrowserWindow): void {
+const UPDATE_REPO =
+  process.env.J621_UPDATE_REPO?.trim() ||
+  "https://gitea.rainbow-herring.ts.net/jakebreath/j621";
+
+interface ReleaseAsset {
+  name: string;
+  browser_download_url: string;
+}
+
+interface Release {
+  tag_name: string;
+  draft: boolean;
+  prerelease: boolean;
+  assets?: ReleaseAsset[];
+}
+
+function releaseVersion(tag: string): [number, number, number] | null {
+  const match = /^desktop-v(\d+)\.(\d+)\.(\d+)$/.exec(tag);
+  if (!match) return null;
+  return [Number(match[1]), Number(match[2]), Number(match[3])];
+}
+
+function compareVersions(
+  a: [number, number, number],
+  b: [number, number, number],
+): number {
+  for (let index = 0; index < 3; index += 1) {
+    if (a[index] !== b[index]) return a[index] - b[index];
+  }
+  return 0;
+}
+
+/**
+ * Resolve the generic feed base electron-updater should use.
+ *
+ * Gitea's API path is case-sensitive (owner/repo must match the login), while
+ * the asset URLs it returns are canonical, so the base is derived from the
+ * platform's metadata asset (`latest-linux.yml` / `latest.yml`).
+ */
+async function resolveReleaseFeed(): Promise<string> {
   const override = process.env.J621_UPDATE_URL?.trim();
-  if (override) autoUpdater.setFeedURL({ provider: "generic", url: override });
+  if (override) return override;
+  const match = /^(https?:\/\/[^/]+)\/([^/]+)\/([^/]+?)\/?$/.exec(UPDATE_REPO);
+  if (!match) {
+    throw new Error(
+      `J621_UPDATE_REPO must be <origin>/<owner>/<repo> (got ${UPDATE_REPO}).`,
+    );
+  }
+  const [, origin, owner, repo] = match;
+  const response = await fetch(
+    `${origin}/api/v1/repos/${owner}/${repo}/releases?limit=50`,
+    { headers: { Accept: "application/json" } },
+  );
+  if (!response.ok) {
+    throw new Error(`Release lookup on Gitea failed (HTTP ${response.status}).`);
+  }
+  const releases = (await response.json()) as Release[];
+  const assetName =
+    process.platform === "win32" ? "latest.yml" : "latest-linux.yml";
+  let best: { version: [number, number, number]; asset: ReleaseAsset } | null =
+    null;
+  for (const release of releases) {
+    if (release.draft || release.prerelease) continue;
+    const version = releaseVersion(release.tag_name);
+    if (!version) continue;
+    const asset = release.assets?.find((entry) => entry.name === assetName);
+    if (!asset) continue;
+    if (!best || compareVersions(version, best.version) > 0) {
+      best = { version, asset };
+    }
+  }
+  if (!best) {
+    throw new Error(`No ${assetName} asset found in ${UPDATE_REPO} releases.`);
+  }
+  return best.asset.browser_download_url.replace(/\/[^/]*$/, "");
+}
+
+function setUpUpdates(win: BrowserWindow): void {
   if (!app.isPackaged) autoUpdater.forceDevUpdateConfig = true;
   autoUpdater.autoDownload = false;
   autoUpdater.autoInstallOnAppQuit = false;
@@ -336,6 +412,8 @@ async function checkForUpdates(win: BrowserWindow): Promise<void> {
   if (updateCheckRunning) return;
   updateCheckRunning = true;
   try {
+    const feed = await resolveReleaseFeed();
+    autoUpdater.setFeedURL({ provider: "generic", url: feed });
     await autoUpdater.checkForUpdates();
   } catch (error) {
     const message = error instanceof Error ? error.message : String(error);