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/config.js

@@ -0,0 +1,202 @@
+/**
+ * Application Configuration
+ * Centralized configuration values for the markdown editor
+ */
+
+const Config = {
+    // ===== TIMING CONFIGURATION =====
+    
+    /**
+     * Long-press threshold in milliseconds
+     * Used for drag-and-drop detection in file tree
+     */
+    LONG_PRESS_THRESHOLD: 400,
+    
+    /**
+     * Debounce delay in milliseconds
+     * Used for editor preview updates
+     */
+    DEBOUNCE_DELAY: 300,
+    
+    /**
+     * Toast notification duration in milliseconds
+     */
+    TOAST_DURATION: 3000,
+    
+    /**
+     * Mouse move threshold in pixels
+     * Used to detect if user is dragging vs clicking
+     */
+    MOUSE_MOVE_THRESHOLD: 5,
+    
+    // ===== UI CONFIGURATION =====
+    
+    /**
+     * Drag preview width in pixels
+     * Width of the drag ghost image during drag-and-drop
+     */
+    DRAG_PREVIEW_WIDTH: 200,
+    
+    /**
+     * Tree indentation in pixels
+     * Indentation per level in the file tree
+     */
+    TREE_INDENT_PX: 12,
+    
+    /**
+     * Toast container z-index
+     * Ensures toasts appear above other elements
+     */
+    TOAST_Z_INDEX: 9999,
+    
+    /**
+     * Minimum sidebar width in pixels
+     */
+    MIN_SIDEBAR_WIDTH: 150,
+    
+    /**
+     * Maximum sidebar width as percentage of container
+     */
+    MAX_SIDEBAR_WIDTH_PERCENT: 40,
+    
+    /**
+     * Minimum editor width in pixels
+     */
+    MIN_EDITOR_WIDTH: 250,
+    
+    /**
+     * Maximum editor width as percentage of container
+     */
+    MAX_EDITOR_WIDTH_PERCENT: 70,
+    
+    // ===== VALIDATION CONFIGURATION =====
+    
+    /**
+     * Valid filename pattern
+     * Only lowercase letters, numbers, underscores, and dots allowed
+     */
+    FILENAME_PATTERN: /^[a-z0-9_]+(\.[a-z0-9_]+)*$/,
+    
+    /**
+     * Characters to replace in filenames
+     * All invalid characters will be replaced with underscore
+     */
+    FILENAME_INVALID_CHARS: /[^a-z0-9_.]/g,
+    
+    // ===== STORAGE KEYS =====
+    
+    /**
+     * LocalStorage keys used throughout the application
+     */
+    STORAGE_KEYS: {
+        /**
+         * Dark mode preference
+         */
+        DARK_MODE: 'darkMode',
+        
+        /**
+         * Currently selected collection
+         */
+        SELECTED_COLLECTION: 'selectedCollection',
+        
+        /**
+         * Last viewed page (per collection)
+         * Actual key will be: lastViewedPage:{collection}
+         */
+        LAST_VIEWED_PAGE: 'lastViewedPage',
+        
+        /**
+         * Column dimensions (sidebar, editor, preview widths)
+         */
+        COLUMN_DIMENSIONS: 'columnDimensions'
+    },
+    
+    // ===== EDITOR CONFIGURATION =====
+    
+    /**
+     * CodeMirror theme for light mode
+     */
+    EDITOR_THEME_LIGHT: 'default',
+    
+    /**
+     * CodeMirror theme for dark mode
+     */
+    EDITOR_THEME_DARK: 'monokai',
+    
+    /**
+     * Mermaid theme for light mode
+     */
+    MERMAID_THEME_LIGHT: 'default',
+    
+    /**
+     * Mermaid theme for dark mode
+     */
+    MERMAID_THEME_DARK: 'dark',
+    
+    // ===== FILE TREE CONFIGURATION =====
+    
+    /**
+     * Default content for new files
+     */
+    DEFAULT_FILE_CONTENT: '# New File\n\n',
+    
+    /**
+     * Default filename for new files
+     */
+    DEFAULT_NEW_FILENAME: 'new_file.md',
+    
+    /**
+     * Default folder name for new folders
+     */
+    DEFAULT_NEW_FOLDERNAME: 'new_folder',
+    
+    // ===== WEBDAV CONFIGURATION =====
+    
+    /**
+     * WebDAV base URL
+     */
+    WEBDAV_BASE_URL: '/fs/',
+    
+    /**
+     * PROPFIND depth for file tree loading
+     */
+    PROPFIND_DEPTH: 'infinity',
+    
+    // ===== DRAG AND DROP CONFIGURATION =====
+    
+    /**
+     * Drag preview opacity
+     */
+    DRAG_PREVIEW_OPACITY: 0.8,
+    
+    /**
+     * Dragging item opacity
+     */
+    DRAGGING_OPACITY: 0.4,
+    
+    /**
+     * Drag preview offset X in pixels
+     */
+    DRAG_PREVIEW_OFFSET_X: 10,
+    
+    /**
+     * Drag preview offset Y in pixels
+     */
+    DRAG_PREVIEW_OFFSET_Y: 10,
+    
+    // ===== NOTIFICATION TYPES =====
+    
+    /**
+     * Bootstrap notification type mappings
+     */
+    NOTIFICATION_TYPES: {
+        SUCCESS: 'success',
+        ERROR: 'danger',
+        WARNING: 'warning',
+        INFO: 'primary'
+    }
+};
+
+// Make Config globally available
+window.Config = Config;
+