Captures page: the recording checklist with one start/end button

Steps come from a bundled copy of contracts checklists/ (assets/contracts/,
refreshed by tool/sync-contracts.sh), not from Dart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

tool/sync-contracts.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+# Refresh the bundled copies of contracts files under assets/contracts/ from a
+# contracts checkout (default: ../contracts). The copies must stay byte-identical:
+# crosscheck fails the build otherwise. New files are added by copying them in once.
+set -eu
+here=$(cd "$(dirname "$0")/.." && pwd)
+src=$(cd "${1:-$here/../contracts}" && pwd)
+cd "$here"
+find assets/contracts -type f | while read -r f; do
+  cp "$src/${f#assets/contracts/}" "$f"
+  echo "synced $f"
+done