Add cost and duration metrics to homepage Task Status section

The homepage Task Status section currently shows counts (Open, In Progress, Review, Approved, Done). Add two additional metric cards showing total cost and total work duration.

Design: Add two more cards to the stats-grid:

• "Cost" - shows total cost in dollars (e.g., "$12.34")
• "Duration" - shows total duration in human-readable format (e.g., "2h 15m")

Implementation:

1. Add fields to HomePage data type: data HomePage = HomePage TaskCore.TaskStats [TaskCore.Task] [TaskCore.Task] Bool AggregatedMetrics UTCTime (add AggregatedMetrics parameter)

2. In homeHandler, compute global aggregated metrics:

• Query all activities for all tasks (not just descendants of an epic)
• Sum cost_cents and duration from all completed activities
• Create helper: getGlobalAggregatedMetrics :: IO AggregatedMetrics

3. In HomePage ToHtml instance, add two stat cards after Done:

• Format cost: (aggTotalCostCents metrics) / 100 as "$X.XX"
• Format duration: convert aggTotalDurationSeconds to "Xh Ym" format
• Use neutral badge class (no status color), maybe "badge-neutral" or just gray

4. Add "badge-neutral" CSS if needed - gray background, dark text

The cards should not be clickable (no href) since they're aggregate metrics, not filterable categories.

Files: Omni/Jr/Web.hs (HomePage type, homeHandler, ToHtml instance), Omni/Task/Core.hs (getGlobalAggregatedMetrics)