refactor: separate CSS and JavaScript into external files

- Extract inline CSS styles to css/styles.css
  - Glass morphism effects (.glass, .dark .glass)
  - Iridescent background gradients
  - Fluid blob shape animations

- Extract all JavaScript to js/main.js
  - Theme toggle functionality with localStorage persistence
  - Smooth scroll navigation functions
  - Modal open/close management
  - Keyboard event handlers (ESC to close modals)
  - Comprehensive JSDoc comments for maintainability

- Update index.html with external file references
- Remove legacy theme-toggle.js and language-toggle.js
- Improve cacheability and maintainability
- Reduce inline code bloat in HTML (60KB → ~950 bytes JS link)

css/styles.css

@@ -0,0 +1,37 @@
+/* Glass Morphism Effect */
+.glass {
+  background: rgba(255, 255, 255, 0.4);
+  backdrop-filter: blur(12px);
+  -webkit-backdrop-filter: blur(12px);
+  border: 1px solid rgba(255, 255, 255, 0.3);
+}
+
+.dark .glass {
+  background: rgba(30, 20, 40, 0.4);
+  border: 1px solid rgba(255, 255, 255, 0.1);
+}
+
+/* Iridescent Background Gradient */
+.iridescent-bg {
+  background:
+    radial-gradient(
+      circle at 0% 0%,
+      rgba(140, 43, 238, 0.15) 0%,
+      transparent 50%
+    ),
+    radial-gradient(
+      circle at 100% 100%,
+      rgba(255, 182, 255, 0.2) 0%,
+      transparent 50%
+    ),
+    radial-gradient(
+      circle at 50% 50%,
+      rgba(140, 43, 238, 0.05) 0%,
+      transparent 100%
+    );
+}
+
+/* Fluid Blob Shapes */
+.fluid-shape {
+  border-radius: 60% 40% 30% 70% / 60% 30% 70% 40%;
+}