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/stores/schedule.js

@@ -10,6 +10,7 @@ export const useScheduleStore = defineStore('schedule', () => {
   const isPaused = ref(false)
   const blockStartedAt = ref(null)    // Date.now() ms when current counting period started
   const blockElapsedOffset = ref(0)   // seconds already elapsed before blockStartedAt
+  const blockElapsedCache = ref({})   // blockId → total elapsed seconds (survives block switches)
   const dayStartTime = ref(null)      // "HH:MM:SS" string or null
   const dayEndTime = ref(null)        // "HH:MM:SS" string or null
 
@@ -39,9 +40,10 @@ export const useScheduleStore = defineStore('schedule', () => {
     if (snapshot.child) child.value = snapshot.child
     dayStartTime.value = snapshot.day_start_time || null
     dayEndTime.value = snapshot.day_end_time || null
-    // Restore elapsed time from server-computed value
+    // Restore elapsed time from server-computed value and seed the per-block cache
     const serverElapsed = snapshot.block_elapsed_seconds || 0
     if (snapshot.session?.current_block_id && serverElapsed > 0) {
+      blockElapsedCache.value[snapshot.session.current_block_id] = serverElapsed
       blockElapsedOffset.value = serverElapsed
       // Only start the live counter if the block is actually running (not paused)
       blockStartedAt.value = isPaused.value ? null : Date.now()
@@ -68,21 +70,25 @@ export const useScheduleStore = defineStore('schedule', () => {
       isPaused.value = false
       blockStartedAt.value = null
       blockElapsedOffset.value = 0
+      blockElapsedCache.value = {}
       dayStartTime.value = null
       dayEndTime.value = null
       return
     }
-    // Pause — accumulate elapsed, stop counting
+    // Pause — accumulate elapsed, save to cache, stop counting
     if (event.event === 'pause') {
       if (blockStartedAt.value) {
         blockElapsedOffset.value += Math.floor((Date.now() - blockStartedAt.value) / 1000)
       }
+      if (event.block_id) {
+        blockElapsedCache.value[event.block_id] = blockElapsedOffset.value
+      }
       blockStartedAt.value = null
       isPaused.value = true
     }
-    // Start (new block) — reset elapsed, begin counting
+    // Start — restore cached elapsed if returning to a previously worked block
     if (event.event === 'start') {
-      blockElapsedOffset.value = 0
+      blockElapsedOffset.value = blockElapsedCache.value[event.block_id] || 0
       blockStartedAt.value = Date.now()
       isPaused.value = false
     }