Add Markdown export v2

[Laksh-star]

Summary

Adds a v2 Markdown export format for timeline range exports from Settings.

The export now includes:

• YAML frontmatter with source, export kind, date range, version, and card count
• A range summary with total activities, estimated tracked time, top category, and top app/site
• Category and app/site rollups
• A weekly rollup when exporting five or more days
• Daily timeline sections with tags, category/subcategory, app/site metadata, summaries, details, and timelapse references

Why

The existing Settings export emits one basic Markdown section per day. This change makes exported timelines more useful for archiving, Notion-style note systems, and later analysis by AI tools while keeping the feature scoped to the existing export surface.

Validation

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer xcodebuild -project Dayflow/Dayflow.xcodeproj -scheme Dayflow -configuration Debug -derivedDataPath DerivedData CODE_SIGNING_ALLOWED=NO build passed.
• xcodebuild test ... CODE_SIGNING_ALLOWED=NO built and ran the unit tests that executed successfully, then failed in the existing UI test runner bundle setup because DayflowUITests could not load its executable. No Markdown export compile or unit-test failure was reported.