Add executive summary export (Markdown)

[romahawk]

Part of #60

What

Let the user copy or download the AI-enhanced retrospective as a Markdown file for use in notes or team updates.

Tasks

• Add "Export Markdown" button to Journal Module weekly review
• Combine user-edited text + AI pattern insights into a structured Markdown template
• Provide clipboard-copy option (primary) and file-download fallback
• Template: ## Week of [date], ### Wins, ### Friction, ### Next Week, ### AI Patterns

AC

• Clipboard copy works in-browser without additional permissions
• Downloaded file named retro-YYYY-WW.md uses ISO week number