feat: period report export (#22)

[wiseelf]

Summary

• Adds ReportGenerator (pure logic in TimeTrackerCore) that produces per-day rows
  with tracked hours, on-call hours, and amounts (when income tracking is enabled)
• Adds ReportPickerSheet — a period selector sheet shown before the save dialog
• Report button in Stats tab opens the picker; Export/Import (JSON backup) moved to
  On-call Settings page
• Saves a Markdown .md file via NSSavePanel with a period-based filename

Period options

Option	Example filename
This Week	time-report-jun-23-29-2026.md
Last Week	time-report-jun-16-22-2026.md
This Month	time-report-june-2026.md
Last Month	time-report-may-2026.md
Custom Range	time-report-jun-1-jun-30-2026.md

Report format

# Time Report — Jun 23–29, 2026

| Date       | Hours | Amount  | On-call | OC Amount |
|------------|------:|--------:|--------:|----------:|
| Mon Jun 23 |  7:30 | $120.00 |    8:00 |    $48.00 |
| Tue Jun 24 |  8:00 | $128.00 |       — |         — |

Regular:   15:30   $248.00
On-call:    8:00    $48.00
Total:              $296.00

Amount columns appear only when income tracking is enabled. On-call columns are
optional via a toggle in the picker. Days with no tracked time and no on-call are omitted.

New files

• Sources/TimeTrackerCore/ReportGenerator.swift — ReportRow, ReportData, ReportGenerator.generate
• Sources/TimeTracker/ReportPickerSheet.swift — period picker sheet (5 options + custom date range + on-call toggle)

Modified files

• Sources/TimeTracker/StatsView.swift — Report button opens picker; Export/Import removed
• Sources/TimeTracker/OnCallSettingsSection.swift — Export/Import (JSON backup) moved here

Test plan

• swift build clean
• Stats tab: Report button opens period picker
• All 5 period options produce correct date ranges and filenames
• Custom range: start/end date pickers enforce start ≤ end
• Include on-call toggle: off removes On-call/OC Amount columns and on-call totals
• Income tracking off: no Amount/OC Amount columns
• Days with no activity are omitted
• On-call Settings: Export/Import work correctly from new location
• Saved .md file opens in any Markdown viewer