Split files into CSS/JS/HTML + add mobile menu

- Extracted CSS to css/style.css (978 lines)
- Extracted JS to js/main.js (708 lines)
- index.html now only 608 lines (was 2289)
- Added mobile hamburger menu for navigation
- Mobile menu toggles with animated icon
- Menu closes when clicking links or outside

partials/head.html

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+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>ENDFIELD // Operator Showcase</title>
+    <link rel="preconnect" href="https://fonts.googleapis.com">
+    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
+    <link href="https://fonts.googleapis.com/css2?family=Orbitron:wght@400;500;600;700;800;900&family=Rajdhani:wght@300;400;500;600;700&family=Share+Tech+Mono&display=swap" rel="stylesheet">

partials/header.html

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+    <!-- HUD Navigation -->
+    <nav class="hud-nav">
+        <div class="nav-logo">ENDFIELD</div>
+        <div class="nav-links">
+            <a href="#hero">Laevatain</a>
+            <a href="#yvonne">Yvonne</a>
+            <a href="#last-rite">Last Rite</a>
+            <a href="#snowshine">Snowshine</a>
+            <a href="#tier-list">Tier List</a>
+            <a href="#team-builder">Team Builder</a>
+            <a href="#game-guide">Game Guide</a>
+        </div>
+    </nav>

partials/hero.html

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+    <!-- Hero - Laevatain -->
+    <section class="hero" id="hero">
+        <img src="characters/Laevatain_Splash.png" alt="Laevatain" class="hero-character">
+        <div class="hero-content">
+            <div class="hero-badge">SS TIER // HEAT ELEMENT</div>
+            <h1><span class="outline">LAEVATAIN</span></h1>
+            <p>"Twilight of ruin!" - Best DPS in Arknights Endfield. Triggers Arts Bursts constantly for devastating damage.</p>
+            <div class="scroll-indicator">↓</div>
+        </div>
+    </section>