feat: add capacity-planner route with demand bands (#207)

[iamasx]

Summary

• Adds a new capacity-planner route with demand bands, utilization cards, and planning notes
• Creates mock data for 4 demand bands, 5 utilization resources, and 4 planning notes
• Builds 3 sub-components: DemandBandCard, UtilizationCardItem, and PlanningNotes
• Adds route-specific CSS module with dark theme styling
• Adds nav link in root layout

Test plan

• 5 tests covering hero/metadata, summary stats, demand bands, utilization cards, and planning notes
• Verify responsive layout across common breakpoints
• Visual review of demand band level badges and utilization status badges

Closes #207

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