feat: Reinitialize frontend with SvelteKit and TypeScript

- Delete old Vite+Svelte frontend
- Initialize new SvelteKit project with TypeScript
- Configure Tailwind CSS v4 + DaisyUI
- Implement JWT authentication with auto-refresh
- Create login page with form validation (Zod)
- Add protected route guards
- Update Docker configuration for single-stage build
- Add E2E tests with Playwright (6/11 passing)
- Fix Svelte 5 reactivity with $state() runes

Known issues:
- 5 E2E tests failing (timing/async issues)
- Token refresh implementation needs debugging
- Validation error display timing

frontend/node_modules/dom-helpers/esm/isVisible.js

@@ -0,0 +1,8 @@
+/**
+ * Checks if a given element is currently visible.
+ * 
+ * @param node the element to check
+ */
+export default function isVisible(node) {
+  return node ? !!(node.offsetWidth || node.offsetHeight || node.getClientRects().length) : false;
+}