chore: add TUI screenshots via VHS

Screenshots capture 4 views: monitors dashboard (hero), detail panel,
alerts tab, and logs tab. Includes VHS tape, demo seed config, and
setup script for reproducible captures.

Also fixes latencySparkline to color DOWN checks red instead of green
— previously failed checks with 0ms latency rendered as green bars.

vhs/setup.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# VHS screenshot setup: seed monitors, backfill history, start server.
+set -e
+DB="${1:?usage: setup.sh <db-path>}"
+
+rm -f "$DB" "$DB-shm" "$DB-wal"
+
+echo "==> Seeding monitors and alerts..."
+UPTOP_DB_DSN="$DB" ./uptop apply -f vhs/seed.yaml 2>&1
+
+echo "==> Backfilling check history..."
+BACKFILL_OUT=$(go run ./vhs/backfill/ "$DB")
+echo "$BACKFILL_OUT"
+
+PUSH_TOKEN=$(echo "$BACKFILL_OUT" | grep '^PUSH_TOKEN=' | cut -d= -f2)
+if [ -n "$PUSH_TOKEN" ]; then
+  echo "==> Sending push heartbeat in 15s (background)..."
+  (sleep 15 && curl -s "http://localhost:18099/api/push" -H "Authorization: Bearer $PUSH_TOKEN" > /dev/null 2>&1) &
+fi
+
+echo "==> Starting uptop server..."
+exec env \
+  UPTOP_DB_DSN="$DB" \
+  UPTOP_PORT=23299 \
+  UPTOP_HTTP_PORT=18099 \
+  UPTOP_ALLOW_PRIVATE_TARGETS=true \
+  ./uptop serve 2>/dev/null