fix: implement robust undo/redo history and clear document action

[ThisIs-Developer]

Overview

This PR resolves critical issues with the editor's toolbar actions and event handling, implementing professional-grade support for Undo, Redo, and Clear Document across web, desktop, and mobile platforms.

Key Changes

1. Tab-Aware State History (Undo & Redo):
   • Introduced a lightweight history engine that tracks values and selections (value, selectionStart, selectionEnd) independently per tab in a global tabHistories dictionary.
   • Intercepts default key shortcuts (Ctrl+Z / Ctrl+Shift+Z / Ctrl+Y) and routes them to custom state restoration when the editor is focused.
   • Groups manual keystrokes intelligently and commits states on word boundaries (space), line breaks, pastes, or programmatic additions.
   • Synchronizes toolbar button disabled states dynamically based on stack history.
2. Clear Document Action:
   • Replaced "Clear Markdown formatting" button and modal with "Clear document", which clears all text inside the active document.
   • Clears content using replaceEditorRange after pushing to the active tab's custom undo stack, allowing full recovery of the cleared document via Undo (Ctrl+Z).
   • Forces immediate, synchronous visual updates of renderMarkdown(), updateLineNumbers(), updateFindHighlights(), and saveCurrentTabState() to update the preview pane, line number gutter, and background syntax highlights instantly without requiring key presses or timeout delays.

Verification

• Stacks are fully isolated per tab.
• History states can be fully undone/redone without desynchronization or cursor drift.
• Cleared documents are fully restorable via Undo (Ctrl+Z).
• No regressions on markdown rendering, themes, settings, or other toolbar options.

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview, Comment	Jun 1, 2026 6:19pm

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