[bugfix] harden countdown snapshot serialization and error messaging

- guard instant-to-epoch conversion against pre-boot instants
- serialize snapshot duration as milliseconds (u128) in DTO/commands
- derive Display/Error for CountdownError and return user-facing error strings
- remove unused command result alias and minor snapshot lock cleanup

src-tauri/src/app_state.rs

@@ -17,7 +17,10 @@ impl ClockAnchor {
     }
 
     pub fn instant_to_epoch_ms(&self, instant: tokio::time::Instant) -> u128 {
-        instant.duration_since(self.boot_instant).as_millis() + self.boot_epoch_ms
+        match instant.checked_duration_since(self.boot_instant) {
+            Some(duration) => duration.as_millis() + self.boot_epoch_ms,
+            None => return 0,
+        }
     }
 }
 //TODO: implement the handling of multiple countdowns

src-tauri/src/countdown/commands.rs

@@ -4,15 +4,13 @@ use crate::AppState;
 use tauri::{command, State};
 use tokio::time::Instant;
 
-type CmdResult<T> = Result<T, String>;
-
 #[command]
 pub async fn countdown_start(state: State<'_, AppState>) -> Result<(), String> {
     state
         .countdown_service
         .start(Instant::now())
         .await
-        .map_err(|e: CountdownError| format!("{e:?}"))?;
+        .map_err(|e: CountdownError| e.to_string())?;
     Ok(())
 }
 
@@ -28,7 +26,7 @@ pub async fn countdown_pause(state: State<'_, AppState>) -> Result<(), String> {
         .countdown_service
         .pause(Instant::now())
         .await
-        .map_err(|e: CountdownError| format!("{e:?}"))?;
+        .map_err(|e: CountdownError| e.to_string())?;
     Ok(())
 }
 
@@ -38,7 +36,7 @@ pub async fn countdown_resume(state: State<'_, AppState>) -> Result<(), String>
         .countdown_service
         .resume(Instant::now())
         .await
-        .map_err(|e: CountdownError| format!("{e:?}"))?;
+        .map_err(|e: CountdownError| e.to_string())?;
     Ok(())
 }
 
@@ -58,7 +56,7 @@ pub async fn countdown_snapshot(
     Ok(CountdownSnapshotDto {
         id: countdown_snapshot.id,
         label: countdown_snapshot.label,
-        duration: countdown_snapshot.duration,
+        duration: countdown_snapshot.duration.as_millis(),
         state: countdown_snapshot.state,
         start_epoch_ms: start,
         target_epoch_ms: target,

src-tauri/src/countdown/dto.rs

@@ -1,12 +1,10 @@
-use tokio::time::Duration;
-
 use crate::countdown::model::CountdownState;
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash, serde::Serialize, serde::Deserialize)]
 pub struct CountdownSnapshotDto {
     pub id: u64,
     pub label: String,
-    pub duration: Duration,
+    pub duration: u128,
     pub state: CountdownState,
     pub start_epoch_ms: Option<u128>,
     pub target_epoch_ms: Option<u128>,
@@ -16,7 +14,7 @@ impl CountdownSnapshotDto {
     pub fn new(
         id: u64,
         label: String,
-        duration: Duration,
+        duration: u128,
         state: CountdownState,
         start_epoch_ms: Option<u128>,
         target_epoch_ms: Option<u128>,

src-tauri/src/countdown/errors.rs

@@ -1,10 +1,13 @@
 use crate::countdown::model::CountdownState;
+use thiserror::Error;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, Error)]
 pub enum CountdownError {
+    #[error("Invalid transition from {from:?} to {action:?}")]
     InvalidTransition {
         from: CountdownState,
         action: &'static str,
     },
+    #[error("Time overflow")]
     TimeOverflow,
 }

src-tauri/src/countdown/service.rs

@@ -30,7 +30,7 @@ impl CountdownService {
     }
 
     pub async fn snapshot(&self, now: Instant) -> CountdownSnapshot {
-        let countdown = self.countdown.lock().await.clone();
+        let countdown = self.countdown.lock().await;
         CountdownSnapshot {
             id: countdown.id(),
             label: countdown.label().to_string(),