feat(config): expose context_window in example config and web UI

Add context_window to config.example.json, the web configuration page
(form model, input field, save handler), and i18n strings (en/zh).
The field is optional — leaving it empty falls back to the 4x max_tokens
heuristic.

config/config.example.json

@@ -5,6 +5,7 @@
       "restrict_to_workspace": true,
       "model_name": "gpt-5.4",
       "max_tokens": 8192,
+      "context_window": 131072,
       "temperature": 0.7,
       "max_tool_iterations": 20,
       "summarize_message_threshold": 20,