refactor: move allow_chat_override to top-level config and improve datetime parsing

- Move allow_chat_override from chat_model to top-level configuration
- Update all references to use new config path
- Set default to true in default_config.yaml, false in migration
- Fix datetime parsing to handle timezone-less ISO strings by appending 'Z'
- Remove inline style from tune icon in config.html

plugins/_model_config/default_config.yaml

@@ -1,3 +1,5 @@
+allow_chat_override: true
+
 chat_model:
   provider: "openrouter"
   name: "anthropic/claude-sonnet-4.6"
@@ -9,7 +11,6 @@ chat_model:
   rl_input: 0
   rl_output: 0
   kwargs: {}
-  allow_chat_override: false
 
 utility_model:
   provider: "openrouter"