feat: redesign editor toolbar and modernize history engine

[ThisIs-Developer]

Redesign Editor Toolbar & Modernize History Engine

Summary

This PR modernizes the markdown viewer's text formatting toolbar and text editing history architecture, replacing deprecated browser execCommand APIs with a custom Virtual History Engine (VHE) and resolving a critical formatting cleanup crash.

Key Changes

• Toolbar Consolidation: Grouped 35+ flat buttons into 7 semantically chunked visual clusters, replacing redundant heading (H1–H6), case conversion, and insert buttons with unified dropdown selectors. Added custom glassmorphism styles and micro-animations.
• Virtual History Engine: Deployed a lightweight string-difference ledger (computeDelta) that tracks character-level and programmatic edits as transactional deltas ($O(1)$ memory). Integrates multi-tab history isolation and a semantic keystroke grouping buffer.
• AST Formatting Cleanup: Coded a robust parser (MarkdownFormattingCleanup) to safely strip markdown tags (headings, lists, code, bold, italics, tables) from range selections or full documents, resolving a critical legacy ReferenceError.
• A11y & Command Palette: Deployed a fuzzy-matching Command Palette overlay (Ctrl+Shift+P) for rapid action execution, and implemented WCAG 2.1 AA roving tabindex arrow-key navigation across toolbar items.

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview, Comment	Jun 1, 2026 5:04pm

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