feat: Support multiple displayTitle sources with fallback

[efirlus]

Summary

• Allow users to specify multiple title sources separated by commas (e.g., #heading,title,name)
• The first non-empty value is used, providing a fallback mechanism
• Fully backward compatible with existing single-value settings

Related Issue

Closes #468

Changes

• src/repositories/documents-repository.ts: Implement comma-separated source parsing with fallback logic
• src/settings/settings-indexing.ts: Update setting description to explain new syntax and add placeholder

Example Usage

Setting	Behavior
title	Same as before - use frontmatter "title"
#heading	Same as before - use first H1 heading
#heading,title	Try H1 first, fallback to frontmatter "title"
#heading,title,name	Try H1, then "title", then "name"

Tested

• Build passes without errors
• Single value settings work as before
• Multiple sources with fallback work correctly
• Whitespace around commas is trimmed properly
• Empty sources are filtered out