feat: add step definitions module and integrate with sidepanel

- Introduced a new module for step definitions in `data/step-definitions.js` to manage shared step metadata.
- Updated `sidepanel.html` to dynamically render steps based on the new step definitions module.
- Refactored `sidepanel.js` to utilize the step definitions for rendering and managing step statuses.
- Enhanced tests to validate the step definitions module and its integration with the sidepanel.
- Cleaned up existing tests to ensure they align with the new structure and functionality.

tests/background-tab-runtime-module.test.js

@@ -0,0 +1,17 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+
+test('background imports tab runtime module', () => {
+  const source = fs.readFileSync('background.js', 'utf8');
+  assert.match(source, /background\/tab-runtime\.js/);
+});
+
+test('tab runtime module exposes a factory', () => {
+  const source = fs.readFileSync('background/tab-runtime.js', 'utf8');
+  const globalScope = {};
+
+  const api = new Function('self', `${source}; return self.MultiPageBackgroundTabRuntime;`)(globalScope);
+
+  assert.equal(typeof api?.createTabRuntime, 'function');
+});