[refactor] simplify app state to a single countdown service

- replace AppState.countdowns vector with countdown_service
- add CountdownService::new(id, label, duration) for configurable initialization
- add CountdownService::default() with default countdown values
- remove unused next_id field and note TODO for multi-countdown support

src-tauri/src/app_state.rs

@@ -20,18 +20,18 @@ impl ClockAnchor {
         instant.duration_since(self.boot_instant).as_millis() + self.boot_epoch_ms
     }
 }
-
+//TODO: implement the handling of multiple countdowns
 #[derive(Debug)]
 pub struct AppState {
     pub clock_anchor: ClockAnchor,
-    pub countdowns: Vec<CountdownService>,
+    pub countdown_service: CountdownService,
 }
 
 impl AppState {
     pub fn new() -> Self {
         Self {
             clock_anchor: ClockAnchor::new(),
-            countdowns: vec![CountdownService::new()],
+            countdown_service: CountdownService::default(),
         }
     }
 }

src-tauri/src/countdown/service.rs

@@ -7,14 +7,16 @@ use crate::countdown::model::{Countdown, CountdownError};
 #[derive(Debug)]
 pub struct CountdownService {
     countdown: Mutex<Countdown>,
-    next_id: u64,
 }
 
 impl CountdownService {
-    pub fn new() -> Self {
+    pub fn default() -> Self {
+        Self::new(0, "Countdown0", Duration::new(600, 0))
+    }
+
+    pub fn new(id: u64, label: &str, duration: Duration) -> Self {
         Self {
-            countdown: Mutex::new(Countdown::new(0, "Countdown0", Duration::new(600, 0))),
-            next_id: 1,
+            countdown: Mutex::new(Countdown::new(id, label, duration)),
         }
     }