feat: add Markdown learning module with 8 beginner lessons

- Add markdown-basics module with lessons for headings, text formatting,
  lists, links, and inline code
- Integrate markdown section with blue color theme (#5b8dd9)
- Add markdown mode support in CodeEditor and LessonEngine
- Add markdown preview rendering using marked library
- Add section overview page with educational content
- Add markdown reference page with syntax guide
- Add i18n translations for 6 languages (EN, DE, PL, ES, AR, UK)
- Update router to recognize #markdown as section route
- Add all section-specific CSS styles for markdown theme

src/index.html

@@ -75,6 +75,7 @@
 						<a href="#css" class="nav-link" data-section="css">CSS</a>
 						<a href="#html" class="nav-link" data-section="html">HTML</a>
 						<a href="#tailwind" class="nav-link" data-section="tailwind">Tailwind</a>
+						<a href="#markdown" class="nav-link" data-section="markdown">Markdown</a>
 						<a href="#reference/css" class="nav-link nav-link-ref" data-section="reference">Reference</a>
 					</nav>
 					<button id="auth-trigger-header" class="btn btn-outline btn-sm" data-i18n="authLogin">Log In</button>
@@ -168,6 +169,12 @@
 								<p data-i18n="landingTailwindDesc">Utility-first CSS framework</p>
 								<span class="section-card-progress" id="tailwind-progress"></span>
 							</a>
+							<a href="#markdown" class="section-card" data-section="markdown">
+								<div class="section-card-icon" style="background: #5b8dd9">MD</div>
+								<h3>Markdown</h3>
+								<p data-i18n="landingMarkdownDesc">Lightweight markup for formatting text</p>
+								<span class="section-card-progress" id="markdown-progress"></span>
+							</a>
 						</div>
 						<p class="device-notice" data-i18n-html="deviceNotice">
 							<strong>Best on desktop or tablet (landscape).</strong> Mobile works, but larger screens make coding easier.
@@ -328,6 +335,7 @@
 						<a href="#reference/flexbox" class="ref-nav-link" data-ref="flexbox">Flexbox</a>
 						<a href="#reference/grid" class="ref-nav-link" data-ref="grid">Grid</a>
 						<a href="#reference/html" class="ref-nav-link" data-ref="html">HTML Elements</a>
+						<a href="#reference/markdown" class="ref-nav-link" data-ref="markdown">Markdown</a>
 					</nav>
 					<div class="reference-body" id="reference-body">
 						<!-- Reference content injected by JS -->