feat(overview): redesign overview page with charts and cost metrics

[r2dedios]

Summary

Closes #270

• Redesign Overview page into operational dashboard with donut chart (clusters by partner), ranked-list cards (cost per account, top regions/owners/partners), and consolidated summary cards
• Add backend endpoints for top regions, top owners, clusters by partner (from tags), and cost per account (from materialized view)
• No database schema changes — all data sourced from existing tables, tags, and materialized views

Changes

Backend

• Add TopItem and AccountCost domain models and DTOs
• Add GetTopRegions, GetTopOwners, GetClustersByPartner to cluster repository
• Add GetCostPerAccount to account repository
• Add QuerySelectContext to DBClient for multi-row raw SQL
• Wire new queries into overview service

Frontend

• Add @patternfly/react-charts v8 with Victory peer dependencies
• New PartnerDonutChart component with theme-aware HTML legend (dark mode compatible)
• New TopMetricCard component with optional currency formatting
• Restructure layout: summary cards row, charts + ranked lists row, events row
• Enlarge provider icons, exclude Terminated from cluster-by-provider total
• Explicit card borders via CSS to override Victory styles

Test data

• Comprehensive load_example_data.sql with 5 accounts, 19 clusters, ~85 instances, 7 partners, 8 owners, 60 days of expenses

Test plan

• make local-build compiles successfully
• make lint — 0 issues
• make console-lint — 0 errors
• make go-unit-tests — all pass
• Visual verification via dev compose with example data
• Verify dark mode readability for all charts and labels
• Verify 5s auto-refresh polling works