feat(ui): redesign Explorer, toolbar icons, and localization

[ThisIs-Developer]

Summary

This PR delivers a complete Markdown Viewer interface refresh focused on the document Explorer, toolbar organization, icon consistency, responsive behavior, and application-wide localization.

The redesign keeps the application lightweight and preserves the existing editor, preview, import, export, sharing, review, storage, and desktop functionality.

Explorer and document management

• Adds a closable left-hand Explorer beside the document tab bar.
• Supports workspaces, nested folders, files, favorites, recent documents, and secret-workspace access.
• Makes complete workspace and folder rows expandable with pointer, Enter, and Space input while maintaining aria-expanded.
• Adds normal, Ctrl/Command, and Shift multi-selection for files and folders.
• Adds context-sensitive right-click and three-dot menus with only supported actions.
• Supports safe bulk deletion with confirmation and drag-and-drop movement between folders and workspaces.
• Keeps closing a tab separate from deleting its document.
• Imports GitHub files into a repository-named folder and displays background import progress.

Tabs and split view

• Adds contextual tab commands for Close, Close Others, Close Left, Close Right, and Close All.
• Supports synchronized two-document split view with one combined split tab.
• Keeps both split documents in the same Edit or Preview mode and synchronizes scrolling.
• Uses compact truncation for long single and split-document names.

Toolbar and icon redesign

• Reorganizes formatting controls by workflow and frequency of use.
• Moves document utilities into logical groups while keeping the header focused on New, Export, Share, Live Share, Report, About, and Settings.
• Standardizes dropdown styling, keyboard dismissal, spacing, and interaction behavior.
• Uses a locally bundled Lucide icon subset with consistent 1.5px strokes and no heavy runtime dependency.
• Retains the recognizable Bootstrap GitHub brand mark as the only icon-system exception.
• Uses compact segmented Edit, Split, and Preview controls.
• Promotes Diagram & More to the main formatting toolbar and simplifies Insert to a three-dot control.

Responsive interface and overlays

• Replaces the floating mobile menu with a compact full-height workspace drawer.
• Organizes mobile View, Document Tools, Actions, and Settings sections with accordion behavior.
• Keeps mobile toolbar and tab-strip sizing aligned with the desktop interface.
• Improves empty-document behavior and disables document-only actions when no document is open.
• Standardizes modal typography, padding, close controls, focus behavior, and responsive layout.
• Refines GitHub import, Share Snapshot, Live Share, Markdown alerts, symbol, emoji, move, rename, and About dialogs.

Localization

• Adds complete interface catalogs for English, Simplified Chinese, Traditional Chinese, Japanese, Korean, Portuguese, Spanish, French, German, Russian, Italian, Turkish, Polish, and Ukrainian.
• Covers 1,029 interface strings per locale, including dynamic dialogs, context menus, Explorer states, editor placeholders, progress messages, validation feedback, titles, and ARIA labels.
• Localizes dynamically created UI through a scoped observer without re-rendering editor content.
• Preserves Markdown source, preview content, file names, user comments, GitHub tree names, language autonyms, and symbolic controls.
• Includes curated terminology for Markdown Viewer, Markdown, and key Korean, Chinese, and German interface labels.
• Loads only the selected locale and retains offline web and desktop support.

Desktop support

• Synchronizes the Neutralino desktop resources with the redesigned web application.
• Includes all locale catalogs in the offline desktop bundle.
• Preserves native file dialogs, storage mirroring, external-file opening, and seven standalone build targets.

User impact

• File and folder organization is easier to discover and operate.
• Toolbar actions are more consistent and less crowded.
• Desktop and mobile layouts share the same interaction language.
• Users can operate the complete interface in their selected supported language.
• No additional heavy UI framework or runtime icon dependency is introduced.

Validation

• Catalog integrity: 1,029 keys across 14 catalogs, with no missing values, mismatched keys, or broken interpolation placeholders.
• Korean and Chinese screenshot-state browser checks: 3 passed.
• Responsive and Explorer regression suites: 16 passed.
• Static translation coverage audit: no untranslated catalogued text nodes, placeholders, titles, or ARIA labels.
• JavaScript syntax checks passed.
• Web static build passed.
• Neutralino desktop build generated all seven standalone targets successfully.
• No tests were rerun for the requested one-commit rollback and PR metadata update.

Commit style

Conventional commit messages are used throughout this branch, including feat, fix, style, refactor, and chore.

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview, Comment	Jul 19, 2026 9:08pm

[ThisIs-Developer]

Superseded by #222 on the renamed branch eature/ui-explorer-toolbar-localization.