Preserve elapsed time when switching between schedule blocks

Previously, clicking a different block always fired 'start' which reset
elapsed to zero — returning to a block lost all accumulated time.

Store changes:
- Add blockElapsedCache (blockId → elapsed seconds) that persists across
  block switches within a session
- On 'pause' WS event: write the block's total elapsed into the cache
- On 'start' WS event: restore elapsed from cache (0 if never worked)
- On applySnapshot: seed cache with server-computed elapsed for the
  current block (so reloading preserves state correctly)
- Clear cache when the session ends

Dashboard selectBlock changes:
- Auto-pause the currently running block before switching to another
- Clicking the active block while paused now sends 'resume' instead of
  doing nothing
- Clicking the already-running active block is a no-op

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

frontend/src/views/DashboardView.vue

@@ -217,10 +217,25 @@ async function sendAction(type) {
   await scheduleStore.sendTimerAction(scheduleStore.session.id, type)
 }
 
-function selectBlock(block) {
+async function selectBlock(block) {
   if (!scheduleStore.session) return
-  scheduleStore.session.current_block_id = block.id
-  scheduleStore.isPaused = false
+
+  const currentId = scheduleStore.session.current_block_id
+
+  // Clicking the current block while paused → resume it
+  if (block.id === currentId && scheduleStore.isPaused) {
+    scheduleStore.sendTimerAction(scheduleStore.session.id, 'resume')
+    return
+  }
+
+  // Clicking the current block while running → do nothing
+  if (block.id === currentId) return
+
+  // Switching to a different block — pause the current one first if it's running
+  if (currentId && !scheduleStore.isPaused) {
+    await scheduleStore.sendTimerAction(scheduleStore.session.id, 'pause', currentId)
+  }
+
   scheduleStore.sendTimerAction(scheduleStore.session.id, 'start', block.id)
 }