fix timers

backend/app/services/pomodoro.py

@@ -176,10 +176,28 @@ def resume(session: Session) -> PomodoroSession:
 
 
 def complete(
-    session: Session, settings: AppSettings, *, auto_next: bool = True
-) -> tuple[PomodoroSession, PomodoroSession | None]:
-    """Finish the current interval and, if configured, roll into the next one."""
-    row = _require_active(session)
+    session: Session,
+    settings: AppSettings,
+    *,
+    auto_next: bool = True,
+    expect_id: int | None = None,
+) -> tuple[PomodoroSession | None, PomodoroSession | None]:
+    """Finish the current interval and, if configured, roll into the next one.
+
+    `expect_id` is the interval the caller believes it is finishing. Every
+    client runs its own local countdown and calls this when it hits zero, so
+    two clients open at once both call within the same second — and the second
+    call used to land on the interval `auto_next` had just created and end it
+    immediately. That is a break disappearing the moment it starts, and it is
+    not something a client can fix on its own: only the server knows which
+    interval is current. So a call that names an interval which is no longer
+    running does nothing at all, and says so by returning no `finished`.
+    """
+    row = current_session(session)
+    if expect_id is not None and (row is None or row.id != expect_id):
+        return None, None
+    if row is None:
+        raise TimerError("No interval is running.")
     finished = _finish(session, settings, row, status="completed")
 
     follow_on: PomodoroSession | None = None