feat(editor): add productivity keyboard shortcuts

[ThisIs-Developer]

What changed

• Add Typora-style Markdown editing shortcuts for bold, italic, strikethrough, inline code, links, clear formatting, paragraph/headings, code blocks, blockquotes, and ordered/unordered lists.
• Add Obsidian-style productivity navigation for Quick Open, Edit/Preview, Explorer, import, settings, Markdown copy, fullscreen, new/close/switch document, and find-result navigation.
• Support both the primary editor and the secondary two-document editor while respecting read-only documents.
• Make toolbar strikethrough and inline-code actions true toggles.
• Fix Shift+Tab outdent and prevent Ctrl/Cmd+Shift+S from also triggering Markdown export.
• Expose shortcuts in tooltips, aria-keyshortcuts, Help, Features, and the Usage Guide.
• Keep browser-reserved tab and heading shortcuts desktop-only where appropriate.

Why

Issue #237 requested keyboard-first Markdown formatting. This expands that fix into a consistent shortcut system based on familiar Typora and Obsidian conventions, while mapping only commands that have a real equivalent in Markdown Viewer.

User impact

The app now documents 35 shortcut categories covering 41 individual editing and application actions. Users can format selections, navigate documents, open app surfaces, and control the editor without leaving the keyboard.

Validation

• node --check script.js
• npm.cmd run build — passed
• Targeted Playwright shortcut tests — 3 passed
• Full editor regression file — 9 passed; 1 known local tab-count fixture failure because an extra stored/release tab is present at startup

Closes #237

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview	Aug 9, 2026 4:46pm

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