fix(modal): compensate scrollbar width when locking page scroll

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/widget-system/modal-scrollbar-compensation.test.cjs

@@ -0,0 +1,15 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const root = path.resolve(__dirname, '../..');
+const runtime = fs.readFileSync(path.join(root, 'assets/js/core/modal-runtime.js'), 'utf8');
+
+test('modal runtime compensates the removed scrollbar like Bootstrap setScrollbar', () => {
+  assert.match(runtime, /var gap = window\.innerWidth - html\.clientWidth;/);
+  assert.match(runtime, /scrollLock\.push\(padRight\(body, gap\)\)/);
+  assert.match(runtime, /var SCROLLBAR_FIXED = "\.nav-fixed \.header, \.float-right";/);
+  assert.match(runtime, /entry\.element\.style\.paddingRight = entry\.inline;/);
+  assert.doesNotMatch(runtime, /classList\.toggle\("aksrt-modal-runtime-open"/);
+});