feat(runtimes): add usage charts, activity heatmap, and hourly distribution

Add comprehensive data visualization to the runtime detail page:
- Daily token usage stacked area chart and daily cost bar chart
- Model distribution donut chart with cost breakdown
- GitHub-style activity heatmap (13 weeks of daily token usage)
- Hourly task distribution bar chart with new backend endpoint
- Responsive 2-column grid layout for charts on wide screens

Backend: new GET /api/runtimes/{runtimeId}/activity endpoint
returning hourly task counts from agent_task_queue.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/pkg/db/queries/runtime_usage.sql

@@ -25,3 +25,10 @@ FROM runtime_usage
 WHERE runtime_id = $1
 GROUP BY provider, model
 ORDER BY provider, model;
+
+-- name: GetRuntimeTaskHourlyActivity :many
+SELECT EXTRACT(HOUR FROM started_at)::int AS hour, COUNT(*)::int AS count
+FROM agent_task_queue
+WHERE runtime_id = $1 AND started_at IS NOT NULL
+GROUP BY hour
+ORDER BY hour;