Fix #16: Add a minimap on the right showing document overview

[github-actions]

Closes #16

Demo

Demo

Summary

Added a minimap panel on the right side of the editor that displays a zoomed-out overview of the entire document. The minimap shows document structure as horizontal bars and includes a viewport indicator showing the current scroll position. Users can click anywhere in the minimap to quickly jump to that section of the document, making navigation in longer notes much easier.

Before & After

Before	After
Editor without minimap - standard view only	Minimap visible on right side showing document overview with viewport indicator

Files Changed

• src/components/Editor.tsx: Added minimapPlugin ViewPlugin that creates a canvas-based minimap with viewport tracking and click-to-jump navigation
• src/components/Editor.css: Added styles for .minimap container, .minimap-canvas, and .minimap-viewport indicator with light/dark theme support

Tests

• e2e/issues/16/verify.spec.ts - Verification test with before/after visual assertions confirming minimap presence
• e2e/issues/16/demo.spec.ts - Demo showing minimap in action with scrolling and click navigation

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