fix: migrate glm-4-flash to glm-4-flash-250414 (model deprecated by Zhipu)

Zhipu AI has deprecated glm-4-flash, causing 404 errors on all chat requests.
Updated all references:
- config: glm-4-flash → glm-4-flash-250414, added glm-z1-flash
- frontend: defaultModel, conversationStore, ChatArea fallback, ModelsAPI

config/chinese-providers.toml

@@ -25,12 +25,19 @@ max_output_tokens = 4096
 supports_streaming = true
 
 [[llm.providers.models]]
-id = "glm-4-flash"
-alias = "GLM-4-Flash"
+id = "glm-4-flash-250414"
+alias = "GLM-4-Flash (免费)"
 context_window = 128000
 max_output_tokens = 4096
 supports_streaming = true
 
+[[llm.providers.models]]
+id = "glm-z1-flash"
+alias = "GLM-Z1-Flash (免费推理)"
+context_window = 128000
+max_output_tokens = 16384
+supports_streaming = true
+
 [[llm.providers.models]]
 id = "glm-4v-plus"
 alias = "GLM-4V-Plus (视觉)"