[FEAT] Add the cycle burndown chart

[martian56]

Summary
The cycles page shows a work item burndown card, but the chart itself is still a placeholder.

Where I saw it

• ui/src/pages/CyclesPage.tsx renders "Burndown chart coming soon." inside the active cycle details panel.
• api/internal/service/cycle.go already returns a completion_chart in the cycle progress response.
• ui already depends on Recharts, so the project has charting available.

Why this matters
Cycle progress is a core planning feature. A placeholder burndown card takes space in the UI but does not help teams understand whether the cycle is on track.

Suggested fix
Use the existing cycle progress response to render a real burndown line chart. If the API data shape is not enough, extend it with the remaining count per day.

Acceptance criteria

• The burndown card renders a chart for cycles with dates and work items.
• The chart handles empty cycles and cycles without dates cleanly.
• The chart updates when work items are completed or removed from the cycle.
• The placeholder text is removed.

Checked on
main at f8e5946