Add manual block start and fix timer display labels

Blocks are now selected without auto-starting the timer. Clicking a block
makes it current (highlighted) but leaves it in a ready state. A "Start"
button (indigo) triggers timing for a fresh block; "Resume" appears for
previously-worked blocks; "Pause" remains while running.

Also fixes the sidebar duration label to show "Done!" when elapsed ≥ total
and "< 1 min" for sub-minute remaining time instead of "0 min".

Backend adds a "select" event type that records an implicit pause for the
previous block, updates current_block_id, and broadcasts is_paused=true
with prev_block_elapsed_seconds so the TV sidebar stays accurate.

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

frontend/src/components/ScheduleBlock.vue

@@ -63,8 +63,9 @@ const durationLabel = computed(() => {
   const totalSec = blockTotalSeconds()
   if (totalSec <= 0) return ''
   const remSec = Math.max(0, totalSec - props.elapsedSeconds)
+  if (remSec <= 0) return 'Done!'
   const remMin = Math.floor(remSec / 60)
-  if (remMin <= 0) return '0 min'
+  if (remMin <= 0) return '< 1 min'
   if (remMin >= 60) {
     const h = Math.floor(remMin / 60)
     const m = remMin % 60