[Feature] Add find and replace functionality

[kom50]

Summary

Search and replace text within the editor.

Requirements

• Open with Ctrl/Cmd + F
• Find input with match highlighting
• Replace input with replace/replace all buttons
• Match case toggle
• Regex toggle
• Match count display (e.g., "3 of 15")
• Navigate between matches (↑/↓ arrows)

UI/UX

• Floating panel at top-right of editor
• Highlight all matches in document
• Current match highlighted differently
• Close with Escape key

Technical Notes

• Use CodeMirror's built-in search addon
• codemirror/addon/search/search.js
• codemirror/addon/search/searchcursor.js

Acceptance Criteria

• User can find text and see matches highlighted
• Can navigate between matches
• Replace single or all occurrences works
• Regex and case-sensitive options work