feat(editor): add Document Outline sidebar

[ThisIs-Developer]

Add Document Outline so readers can navigate Markdown headings while staying in Editor, Preview, or Split mode.

• Place toolbar controls immediately after Emoji in this order: Emoji | Find & Replace | Full Screen | Document Outline, using the existing toolbar styles without a selected effect on the Outline button.
• Show an automatically updated H1–H6 tree with individual parent chevrons, hierarchy guides, wrapping titles, active-section highlighting, and click-to-scroll navigation. Use the File Sidebar's folding pattern for the header's Collapse/Expand All control.
• Share Find & Replace's header structure and styles, with text-only titles in both panels and matching 340px default widths.
• Reuse the File Sidebar's horizontal resize handler and handle styling for Outline and Find & Replace, including floating and docked Find & Replace. Support pointer dragging, arrow-key resizing, Shift for larger steps, and Home/double-click reset. Account for panel widths in Split mode and constrain docked widths to leave document space. Follow the existing mobile pattern by fitting panels to the screen and hiding resize handles.
• Update the translation catalogs, generated desktop resources, README, changelog, and wiki. Remove the outdated outline screenshot from the usage guide.

Validation: Earlier outline changes passed the static build/SEO checks, 60 outline browser/scenario combinations across full runs and focused reruns, and 31 related Chromium regression checks. The latest header, width, and resizing changes have not been tested, as requested by the user. Desktop resources were regenerated for the latest changes.

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Project	Deployment	Actions	Updated
markdown-viwer	Ready	Preview	Sep 23, 2026 5:48pm UTC

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