feat: implement standard-compliant sidebar with comprehensive tests

Implement a new sidebar component based on DaisyUI Drawer pattern without
custom CSS variants. The sidebar supports desktop (expanded/collapsed states)
and mobile (overlay drawer) with full accessibility compliance.

Sidebar Implementation:
- Refactor sidebar component with sidebar_header, menu_item, menu_group,
  sidebar_footer sub-components
- Add logo (mila.svg) with size-8 (32px) always visible
- Implement toggle button with icon swap (chevron-left/right) for desktop
- Add nested menu support with details/summary (expanded) and dropdown
  (collapsed) patterns
- Implement footer with language selector (expanded-only), theme toggle,
  and user menu with avatar
- Update layouts.ex to use drawer pattern with data-sidebar-expanded
  attribute for state management

CSS & JavaScript:
- Add CSS styles for sidebar state management via data-attribute selectors
- Implement SidebarState JavaScript hook for localStorage persistence
- Add smooth width transitions (w-64 ↔ w-16) for desktop collapsed state
- Add CSS classes for expanded-only, menu-label, and icon visibility

Documentation:
- Add sidebar-analysis-current-state.md: Analysis of current implementation
- Add sidebar-requirements-v2.md: Complete specification for new sidebar
- Add daisyui-drawer-pattern.md: DaisyUI pattern documentation
- Add umsetzung-sidebar.md: Step-by-step implementation guide

Testing:
- Add comprehensive component tests for all sidebar sub-components
- Add integration tests for sidebar state management and mobile drawer
- Extend accessibility tests (ARIA labels, roles, keyboard navigation)
- Add regression tests for duplicate IDs, hover effects, and tooltips
- Ensure full test coverage per specification requirements

priv/static/images/mila.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
+  <rect width="100" height="100" rx="12" fill="#4f46e5"/>
+  <text x="50" y="65" font-family="Arial, sans-serif" font-size="48" font-weight="bold" fill="white" text-anchor="middle">M</text>
+</svg>
+