Add OBB transfers, private app-data backups and SideQuest source policy

Borrow expansion-file and save-management features with offline verification. Keep SideQuest page-only under its current access terms; document research, integration limits and device acceptance gaps.

Co-Authored-By: GPT-6 Astra <noreply@openai.com>

ui/apk_sources/sidequest.py

@@ -0,0 +1,41 @@
+"""SideQuest page links only: its terms do not authorise third-party scraping.
+
+No API calls, cached listings or automated downloads. See docs/sidequest.md.
+"""
+from . import SourceError
+
+KIND = 'sidequest'
+URL = 'https://sidequestvr.com'
+REASON = ('SideQuest is page-only: its terms restrict scraping and unauthorised '
+          'access. Browse and download with SideQuest, then import a developer-provided APK.')
+
+
+def sources():
+    return [{'id': KIND, 'kind': KIND, 'name': 'SideQuest', 'url': URL,
+             'builtin': True, 'enabled': True, 'trust': 'community',
+             'page_only': True, 'reason': REASON}]
+
+
+def search(source, query, limit=50):
+    # Do not invent catalogue results or interpret a query as a verified free app.
+    if limit <= 0:
+        return []
+    raise SourceError(REASON + ' ' + URL + '/apps')
+
+
+def details(source, entry_id):
+    entry_id = str(entry_id)
+    if not entry_id.isascii() or not entry_id.isdecimal() or len(entry_id) > 12:
+        raise SourceError('SideQuest listing ids must be numeric')
+    return {'source': source['id'], 'id': entry_id, 'package': None,
+            'name': 'SideQuest listing ' + entry_id, 'summary': REASON,
+            'icon': None, 'page': URL + '/app/' + entry_id, 'version': None,
+            'version_code': None, 'min_sdk': None, 'abis': None, 'vr': None,
+            'size': None, 'free': None, 'license': None, 'updated': None,
+            'downloadable': False, 'versions': [], 'tags': [], 'headsets': [],
+            'images': {'icon': None, 'banner': None, 'screenshots': []}}
+
+
+def download(source, entry_id, version_code=None):
+    entry = details(source, entry_id)
+    raise SourceError(REASON + ' ' + entry['page'])