feat: add formatting toolbar reorder and visibility settings

[j4rviscmd]

Summary

Add a new settings section that allows users to customize the FormattingToolbar (BubbleMenu) by reordering items via drag-and-drop and toggling their visibility.

Changes

New Files

• src/app/providers/toolbar-config-provider.tsx — Context Provider with persistence via tauri-plugin-store
• src/features/settings/lib/toolbarConfig.ts — Type definitions, defaults (13 items), validation for schema evolution
• src/features/settings/ui/ToolbarOption.tsx — Settings UI with @dnd-kit sortable list, visibility toggles, and matching BubbleMenu icons

Modified Files

• src/App.tsx — Wrap AppContent with ToolbarConfigProvider
• src/app/providers/store-provider.tsx — Add toolbarConfig to configDefaults and pre-fetch
• src/features/editor/ui/Editor.tsx — Replace static toolbar builder with Provider-driven useMemo
• src/features/settings/ui/SettingsDialog.tsx — Add Toolbar section
• src/features/settings/index.ts — Export new types and constants

Features

• Drag-and-drop reorder of 13 toolbar items (Bold, Italic, Underline, etc.)
• Show/hide toggle per item with Switch component
• Each item displays its actual BubbleMenu icon for intuitive identification
• Reset to default button when configuration is customized
• Persists to tauri-plugin-store (config.json)
• Schema evolution support via validateToolbarConfig()
• Context-dependent items (blockTypeSelect, file operations, etc.) are always shown and not configurable