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/header.html

@@ -0,0 +1,13 @@
+    <!-- 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>