Fixed manifest updating for smaller url prefix

When the value of the URL prefix would be changed, if the resulting json would be smaller than the current json, it would create invalid json. Now we wipe the contents before writing to avoid this.

backend/settings.py

@@ -5,7 +5,7 @@ Getting and setting settings
 """
 
 import logging
-from json import dumps, loads
+from json import dump, load
 from typing import Any
 
 from backend.custom_exceptions import InvalidKeyValue, KeyNotFound
@@ -67,7 +67,10 @@ def _format_setting(key: str, value):
 		if not isinstance(value, str):
 			raise InvalidKeyValue(key, value)
 
-		if value:
+		if value == '/':
+			value = ''
+
+		elif value:
 			value = '/' + value.strip('/')
 
 	elif key == 'log_level' and not value in (logging.INFO, logging.DEBUG):
@@ -171,12 +174,16 @@ def update_manifest(url_base: str) -> None:
 	Args:
 		url_base (str): The url base to use in the file.
 	"""
-	with open(folder_path('frontend', 'static', 'json', 'pwa_manifest.json'), 'r+') as f:
-		manifest = loads(f.read())
+	filename = folder_path('frontend', 'static', 'json', 'pwa_manifest.json')
+
+	with open(filename, 'r') as f:
+		manifest = load(f)
 		manifest['start_url'] = url_base + '/'
 		manifest['icons'][0]['src'] = f'{url_base}/static/img/favicon.svg'
-		f.seek(0)
-		f.write(dumps(manifest, indent=4))
+
+	with open(filename, 'w') as f:
+		dump(manifest, f, indent=4)
+
 	return
 
 def backup_hosting_settings() -> None:
@@ -228,5 +235,7 @@ def restore_hosting_settings() -> None:
 		"UPDATE config SET value = ? WHERE key = ?",
 		((v, k) for k, v in hosting_settings.items())
 	)
-	
+
+	update_manifest(hosting_settings['url_prefix'])
+
 	return