Add component tests for EndRunModal, GameGrid, RulesConfiguration, Layout

33 tests covering rendering, user interactions (userEvent clicks), prop
callbacks, filter state, and conditional description text. Adds a
matchMedia stub to the vitest setup file so components importing
useTheme don't throw in jsdom. Also adds actionlint and zizmor
pre-commit hooks for GitHub Actions linting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/test/setup.ts

@@ -1 +1,17 @@
 import '@testing-library/jest-dom'
+
+// jsdom does not implement window.matchMedia; provide a minimal stub so
+// modules that reference it at load time (e.g. useTheme) don't throw.
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: vi.fn().mockImplementation((query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+})