[STORY] v4-S21: Implement responsive layout with dark mode

[raifdmueller]

User Story

As a documentation reader, I want a responsive site with dark mode support.

Acceptance Criteria

Given the new theme CSS
When I view the site on desktop (1440px), tablet (768px), and mobile (375px)
Then the layout adapts appropriately at each breakpoint
And a dark mode toggle persists the preference in localStorage
And Lighthouse accessibility score is above 90

Priority: Should

Epic: v4-E4 (#6)

Blocked by: v4-S20

[raifdmueller]

Analysis

Dark Mode Implementation

CSS custom properties approach:
```css
:root { --bg: #fff; --text: #1a1a1a; }
[data-theme="dark"] { --bg: #1a1a1a; --text: #e0e0e0; }
```
Toggle stored in localStorage. Respects prefers-color-scheme media query as default.

Responsive Breakpoints

• Desktop: ≥1280px (sidebar + content + TOC)
• Tablet: 768-1279px (sidebar + content, TOC hidden)
• Mobile: <768px (hamburger menu, content only)

Lighthouse Targets

• Performance: >90
• Accessibility: >90
• Best Practices: >90
• Page load: <1 second

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