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/default-launcher.json

@@ -1,13 +1,26 @@
 {
   "game_dir": "",
   "update_feed_url": "",
+  "launcher_updates_from_github": false,
+  "gitea": {
+    "base_url": "",
+    "owner": "",
+    "repo": "",
+    "release_tag": "latest",
+    "token_env": "GITEA_TOKEN"
+  },
   "github": {
     "owner": "Dawnforger",
-    "repo": "Fractured-Distro",
+    "repo": "Fractured",
     "ref": "main",
     "release_tag": "latest",
     "token_env": "GITHUB_TOKEN"
   },
+  "patch_manifest": {
+    "enabled": true,
+    "source": "patch-manifest.json",
+    "from_release": true
+  },
   "files": [
     {
       "source": "patch-Z.MPQ",