feat: přidáno URL pole pro bloky – klikatelný odkaz v PDF

- Block model: nové volitelné pole 'url' (Optional[str])
- Frontend: URL input v modálu pro přidání/editaci bloku
- PDF generátor: c.linkURL() – celý blok je klikatelný odkaz
- sample.json: ukázkový blok s URL
- index.html: dokumentace URL pole
- .github/copilot-instructions.md: přidány Copilot instrukce

.githooks/pre-commit

@@ -1,14 +1,23 @@
 #!/usr/bin/env bash
 set -euo pipefail
 # Pre-commit hook: run tests and prevent commit on failures.
-# By default run fast tests (exclude integration). To include integration tests set RUN_INTEGRATION=1.
+# By default run fast tests (exclude integration). To include integration tests
+# set RUN_INTEGRATION=1.
+
+# Use venv pytest if available, otherwise fall back to system pytest
+REPO_DIR="$(git rev-parse --show-toplevel)"
+if [ -x "${REPO_DIR}/.venv/bin/pytest" ]; then
+  PYTEST="${REPO_DIR}/.venv/bin/pytest"
+else
+  PYTEST="pytest"
+fi
 
 echo "Running pytest (fast) before commit..."
 if [ "${RUN_INTEGRATION:-0}" = "1" ]; then
   echo "RUN_INTEGRATION=1: including integration tests"
-  pytest -q
+  $PYTEST -q
 else
-  pytest -q -m "not integration"
+  $PYTEST -q -m "not integration"
 fi
 
 echo "Tests passed. Proceeding with commit."