fix(fitness): mirror finish overview to other devices via SSE

Other devices were left on a blank active-workout page when one device
hit Finish. The finish broadcast now carries the saved session document,
and the receiving page builds the completion overview from it. If the
remote end-of-workout has no session payload (cancel, or the finishing
device couldn't post the session), receivers redirect to the workout
home instead of stranding on a blank page.

src/lib/js/workoutSync.svelte.ts

@@ -37,6 +37,7 @@ export function createWorkoutSync() {
 
   let status: SyncStatus = $state('idle');
   let serverVersion = $state(0);
+  let lastFinishedSession: any = $state(null);
   let eventSource: EventSource | null = null;
   let debounceTimer: ReturnType<typeof setTimeout> | null = null;
   let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
@@ -166,8 +167,15 @@ export function createWorkoutSync() {
         } catch {}
       });
 
-      eventSource.addEventListener('finished', () => {
-        // Another device finished the workout
+      eventSource.addEventListener('finished', (e) => {
+        // Another device finished the workout. If they passed along the saved
+        // session, expose it so the active page can build the completion overview.
+        try {
+          const data = JSON.parse(e.data);
+          lastFinishedSession = data?.session ?? null;
+        } catch {
+          lastFinishedSession = null;
+        }
         workout.cancel();
         disconnectSSE();
       });
@@ -217,14 +225,22 @@ export function createWorkoutSync() {
     connectSSE();
   }
 
-  /** Called when workout finishes or is cancelled — clean up server state */
-  async function onWorkoutEnd() {
+  /** Called when workout finishes or is cancelled — clean up server state.
+   *  Pass the just-saved session id so other devices receive it via SSE
+   *  and can render the finish overview. */
+  async function onWorkoutEnd(sessionId?: string | null) {
     disconnectSSE();
     try {
-      await fetch('/api/fitness/workout/active', { method: 'DELETE' });
+      const qs = sessionId ? `?sessionId=${encodeURIComponent(sessionId)}` : '';
+      await fetch(`/api/fitness/workout/active${qs}`, { method: 'DELETE' });
     } catch {}
   }
 
+  /** Clear the finished-session payload after the page has consumed it. */
+  function clearFinishedSession() {
+    lastFinishedSession = null;
+  }
+
   /** Called on app load — reconcile local vs server state */
   async function init() {
     try {
@@ -290,9 +306,11 @@ export function createWorkoutSync() {
   return {
     get status() { return status; },
     get serverVersion() { return serverVersion; },
+    get lastFinishedSession() { return lastFinishedSession; },
     init,
     onWorkoutStart,
     onWorkoutEnd,
+    clearFinishedSession,
     notifyChange,
     destroy
   };