fix(launcher): opt-in GitHub auto-update; clarify Gitea for from_release

- Gate electron-updater GitHub provider on launcher_updates_from_github (default false)
  so GITHUB_TOKEN no longer targets the source repo without latest.yml.
- Improve GitHub releases 404 hint when assets are on Gitea.
- Document in README and default-launcher.json.

Co-authored-by: Cursor <cursoragent@cursor.com>

tools/fractured-launcher-electron/lib/github.js

@@ -3,6 +3,7 @@
 const path = require('path');
 const fs = require('fs').promises;
 const { githubToken } = require('./github-token');
+const { downloadGiteaReleaseAsset, useGiteaReleases } = require('./gitea-release');
 const { fetchToFile, downloadBodyToFile } = require('./http-download');
 
 function encodeRepoPath(repoPath) {
@@ -65,6 +66,9 @@ async function downloadGitHubRepoFile(cfg, repoPath, destPath) {
 }
 
 async function downloadReleaseAsset(cfg, assetName, destPath) {
+  if (useGiteaReleases(cfg)) {
+    return downloadGiteaReleaseAsset(cfg, assetName, destPath);
+  }
   const token = githubToken(cfg);
   const tag = (cfg.github.release_tag || 'latest').trim() || 'latest';
   const { owner, repo } = cfg.github;
@@ -78,7 +82,10 @@ async function downloadReleaseAsset(cfg, assetName, destPath) {
   const text = await res.text();
   if (!res.ok) {
     let hint = '';
-    if (res.status === 404) hint = ' (wrong tag or private repo without token?)';
+    if (res.status === 404) {
+      hint =
+        ' (wrong tag, private repo without token, or releases live on Gitea — set gitea.base_url, gitea.owner, gitea.repo in launcher.json)';
+    }
     if (res.status === 401 || res.status === 403) hint = ' (set GITHUB_TOKEN or token_env PAT)';
     throw new Error(`releases list ${res.status}${hint}: ${text.slice(0, 600)}`);
   }