Fix timer reset on refresh and sync between dashboard and TV view

- Backend computes block_elapsed_seconds server-side from timer_events
- Store tracks blockStartedAt (ms) + blockElapsedOffset (seconds) instead
  of a client-side counter; updated correctly on start/pause/resume/end
- TimerDisplay derives elapsed from store props so both views always agree
- Add compact timer display to dashboard session card
- Add isPaused/pause-resume logic to dashboard Pause/Resume buttons

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

frontend/src/views/TVView.vue

@@ -26,6 +26,9 @@
         <TimerDisplay
           :block="scheduleStore.currentBlock"
           :session="scheduleStore.session"
+          :is-paused="scheduleStore.isPaused"
+          :block-started-at="scheduleStore.blockStartedAt"
+          :block-elapsed-offset="scheduleStore.blockElapsedOffset"
         />
         <div class="tv-block-notes" v-if="scheduleStore.currentBlock.notes">
           {{ scheduleStore.currentBlock.notes }}