Fixes #5: Refactor code quality - Part 1

- Split index.js from 1,567 lines to 456 lines (73% reduction)
  - Extracted settings event listeners to src/systems/ui/settingsListeners.js
  - Extracted settings panel to src/core/settingsPanel.js
  - Simplified initUI() and main initialization block

- Modularized style.css into 28 logical CSS modules in src/styles/
  - Added build script (npm run build:css) to concatenate modules
  - Modules: panel, components, user-stats, info-box, thoughts, settings,
    themes, modals, mobile, inventory, quests, equipment, encounters,
    weather, music-player, FAB widgets, strip widgets, etc.

- Updated package.json with build:css script and type: module

No functional changes - pure refactoring for maintainability.

scripts/build-css.js

@@ -0,0 +1,70 @@
+#!/usr/bin/env node
+/**
+ * CSS Build Script
+ * Concatenates modular CSS files from src/styles/ into style.css
+ *
+ * Usage: node scripts/build-css.js
+ *
+ * File order matters! Files are concatenated in alphabetical order.
+ * Prefix files with numbers to control order: _01_variables.css, _02_panel.css, etc.
+ */
+
+import fs from 'fs-extra';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const rootDir = path.resolve(__dirname, '..');
+const stylesDir = path.join(rootDir, 'src', 'styles');
+const outputFile = path.join(rootDir, 'style.css');
+
+async function buildCSS() {
+    console.log('Building CSS from src/styles/...');
+
+    // Read all CSS files from src/styles/
+    const files = await fs.readdir(stylesDir);
+    const cssFiles = files
+        .filter(f => f.endsWith('.css'))
+        .sort() // Alphabetical order (use _01_, _02_ prefix for control)
+        .map(f => path.join(stylesDir, f));
+
+    if (cssFiles.length === 0) {
+        console.error('No CSS files found in src/styles/');
+        process.exit(1);
+    }
+
+    console.log(`Found ${cssFiles.length} CSS files:`);
+    cssFiles.forEach(f => console.log(`  - ${path.basename(f)}`));
+
+    // Read and concatenate all files
+    const contents = [];
+    for (const file of cssFiles) {
+        const content = await fs.readFile(file, 'utf-8');
+        const basename = path.basename(file);
+        contents.push(`/* ============================================ */`);
+        contents.push(`/* ${basename} */`);
+        contents.push(`/* ============================================ */`);
+        contents.push('');
+        contents.push(content);
+        contents.push('');
+    }
+
+    const combined = contents.join('\n');
+
+    // Write to style.css
+    await fs.writeFile(outputFile, combined, 'utf-8');
+
+    const stats = await fs.stat(outputFile);
+    const lines = combined.split('\n').length;
+    const sizeKB = (stats.size / 1024).toFixed(1);
+
+    console.log(`\n✅ Built ${outputFile}`);
+    console.log(`   Lines: ${lines}`);
+    console.log(`   Size: ${sizeKB} KB`);
+}
+
+buildCSS().catch(err => {
+    console.error('Build failed:', err);
+    process.exit(1);
+});