refactor: Modularize UI components and utilities

- Extract UI components into separate JS files
- Centralize configuration values in config.js
- Introduce a dedicated logger module
- Improve file tree drag-and-drop and undo functionality
- Refactor modal handling to a single manager
- Add URL routing support for SPA navigation
- Implement view mode for read-only access

static/js/dark-mode.js

@@ -0,0 +1,77 @@
+/**
+ * Dark Mode Module
+ * Manages dark mode theme switching and persistence
+ */
+
+class DarkMode {
+    constructor() {
+        this.isDark = localStorage.getItem(Config.STORAGE_KEYS.DARK_MODE) === 'true';
+        this.apply();
+    }
+
+    /**
+     * Toggle dark mode on/off
+     */
+    toggle() {
+        this.isDark = !this.isDark;
+        localStorage.setItem(Config.STORAGE_KEYS.DARK_MODE, this.isDark);
+        this.apply();
+        
+        Logger.debug(`Dark mode ${this.isDark ? 'enabled' : 'disabled'}`);
+    }
+
+    /**
+     * Apply the current dark mode state
+     */
+    apply() {
+        if (this.isDark) {
+            document.body.classList.add('dark-mode');
+            const btn = document.getElementById('darkModeBtn');
+            if (btn) btn.textContent = '☀️';
+
+            // Update mermaid theme
+            if (window.mermaid) {
+                mermaid.initialize({ theme: Config.MERMAID_THEME_DARK });
+            }
+        } else {
+            document.body.classList.remove('dark-mode');
+            const btn = document.getElementById('darkModeBtn');
+            if (btn) btn.textContent = '🌙';
+
+            // Update mermaid theme
+            if (window.mermaid) {
+                mermaid.initialize({ theme: Config.MERMAID_THEME_LIGHT });
+            }
+        }
+    }
+
+    /**
+     * Check if dark mode is currently enabled
+     * @returns {boolean} True if dark mode is enabled
+     */
+    isEnabled() {
+        return this.isDark;
+    }
+
+    /**
+     * Enable dark mode
+     */
+    enable() {
+        if (!this.isDark) {
+            this.toggle();
+        }
+    }
+
+    /**
+     * Disable dark mode
+     */
+    disable() {
+        if (this.isDark) {
+            this.toggle();
+        }
+    }
+}
+
+// Make DarkMode globally available
+window.DarkMode = DarkMode;
+