feat(dashboard): Enhance dashboard with PageHeader, StatCard, and auth fixes

- Create PageHeader component with title, description, and action slots
- Create StatCard component with trend indicators and icons
- Update dashboard with KPI cards, Quick Actions, and Allocation Preview
- Polish login page with branding and centered layout
- Fix auth redirect: authenticated users accessing /login go to dashboard
- Fix page refresh: auth state persists, no blank page
- Fix sidebar: visible after login, toggle works, state persists
- Fix CSS import: add app.css to layout, fix DaisyUI import path
- Fix breadcrumbs: home icon links to /dashboard
- Add comprehensive E2E and unit tests

Refs: openspec/changes/p03-dashboard-enhancement
Closes: p03-dashboard-enhancement

frontend/tests/unit/layout.store.test.ts

@@ -23,7 +23,7 @@ describe('layout store', () => {
     expect(getStoreValue(store.theme)).toBe('light');
   });
 
-  it('toggleSidebar cycles through states', async () => {
+  it('toggleSidebar toggles expanded/collapsed and recovers from hidden', async () => {
     const store = await import('../../src/lib/stores/layout');
 
     store.setSidebarState('expanded');
@@ -32,8 +32,9 @@ describe('layout store', () => {
     expect(localStorage.setItem).toHaveBeenCalledWith('headroom_sidebar_state', 'collapsed');
 
     store.toggleSidebar();
-    expect(getStoreValue(store.sidebarState)).toBe('hidden');
+    expect(getStoreValue(store.sidebarState)).toBe('expanded');
 
+    store.setSidebarState('hidden');
     store.toggleSidebar();
     expect(getStoreValue(store.sidebarState)).toBe('expanded');
   });