fix: colour scheme not persisting & incorrect initialization

[neil-lobo]

Fixed Issues:

1. After changing your colour scheme and restarting the application, your colour scheme would not persist.
2. The selection ring around the currently selected colour scheme was being incorrectly applied
3. Initialization of colour scheme was incorrect

Cause 1:

After the client sends a POST request to this endpoint

lettura/src-tauri/src/server/handlers/common.rs

Lines 16 to 20 in 7a66c92

	#[post("/api/user-config")]
	pub async fn handle_update_user_config(user_cfg: web::Json<config::UserConfig>) -> Result<impl Responder> {
	let result = config::update_user_config(user_cfg.0);
	Ok(web::Json(result))
	}

the payload is deserialized incorrectly due to missing field color_scheme in the UserConfig struct:

lettura/src-tauri/src/core/config.rs

Lines 56 to 68 in 7a66c92

	#[derive(Debug, Serialize, Deserialize)]
	pub struct UserConfig {
	pub threads: i32,
	pub theme: String,
	pub update_interval: u64,
	pub last_sync_time: String,
	pub local_proxy: Option<LocalProxy>,
	pub customize_style: CustomizeStyle,
	pub purge_on_days: u64,
	pub purge_unread_articles: bool
	}

resulting in colour scheme information not being saved.

[zhanglun]

thank you