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/src/lib/components/layout/Sidebar.svelte

@@ -10,6 +10,7 @@
 
 <script lang="ts">
   import { onMount } from 'svelte';
+  import { get } from 'svelte/store';
   import {
     Moon,
     PanelLeftClose,
@@ -26,6 +27,7 @@
     toggleTheme
   } from '$lib/stores/layout';
   import { user } from '$lib/stores/auth';
+  import type { SidebarState } from '$lib/types/layout';
   import SidebarSection from './SidebarSection.svelte';
 
   $: isExpanded = $sidebarState === 'expanded';
@@ -46,13 +48,22 @@
     document.documentElement.setAttribute('data-sidebar', $sidebarState);
   }
 
+  function getSidebarStateForWidth(width: number): SidebarState {
+    if (width < 768) return 'hidden';
+    if (width < 1280) return 'collapsed';
+    return 'expanded';
+  }
+
   onMount(() => {
-    if ($sidebarState === 'expanded') {
-      if (window.innerWidth < 768) {
-        setSidebarState('hidden');
-      } else if (window.innerWidth < 1280) {
-        setSidebarState('collapsed');
-      }
+    if (typeof window === 'undefined') return;
+
+    const currentState = get(sidebarState);
+    const desiredState = getSidebarStateForWidth(window.innerWidth);
+
+    if (window.innerWidth < 768 && currentState !== 'hidden') {
+      setSidebarState('hidden');
+    } else if (currentState === 'hidden' && window.innerWidth >= 768) {
+      setSidebarState(desiredState);
     }
 
     window.addEventListener('keydown', toggleWithShortcut);