feat: make context limit configurable in API settings

Add context limit field to API settings that:
- Stores context limit in config (defaults to 200000)
- Allows users to set custom limits for different models
- Uses configured limit in token counter display
- Shows format like "2.5k / 200k tokens" using actual limit

This allows proper token tracking for non-Claude models with
different context windows.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -20,6 +20,12 @@ struct ApiConfig {
     active_character_id: Option<String>,
     #[serde(default)]
     stream: bool,
+    #[serde(default = "default_context_limit")]
+    context_limit: u32,
+}
+
+fn default_context_limit() -> u32 {
+    200000
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -1161,7 +1167,7 @@ async fn validate_api(base_url: String, api_key: String) -> Result<Vec<String>,
 }
 
 #[tauri::command]
-async fn save_api_config(base_url: String, api_key: String, model: String, stream: bool) -> Result<(), String> {
+async fn save_api_config(base_url: String, api_key: String, model: String, stream: bool, context_limit: u32) -> Result<(), String> {
     // Preserve existing active_character_id if it exists
     let active_character_id = load_config().and_then(|c| c.active_character_id);
 
@@ -1171,6 +1177,7 @@ async fn save_api_config(base_url: String, api_key: String, model: String, strea
         model,
         active_character_id,
         stream,
+        context_limit,
     };
     save_config(&config)
 }