Tie break timer and main block timer together

- Starting break now pauses the main block timer (frontend optimistic +
  backend implicit pause event recorded before break_start)
- Resuming/starting the main block while break is active pauses the
  break timer and exits break mode on all clients including TV
- Timer display counts negative past zero so overtime is visible while
  label stays "Done!"
- Fixed WS start handler incorrectly skipping break-mode clear when
  restarting the same block; resume handler now also clears break mode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/DashboardView.vue

@@ -88,7 +88,7 @@
                 <button
                   class="btn-sm"
                   v-if="scheduleStore.isPaused && scheduleStore.blockElapsedOffset > 0"
-                  @click="sendAction('resume')"
+                  @click="scheduleStore.resumeCurrentBlock(scheduleStore.session.id)"
                 >Resume</button>
                 <button
                   class="btn-sm"