feat: add optional OpenCost cost awareness

[juzigu40-ui]

Closes #49.

Summary

• adds optional [cost] config and OpenCost / Kubecost endpoint discovery
• queries allocation/compute and maps pod cost onto pods plus controller-backed resources
• renders an inline cost column and namespace / cluster aggregate in the header
• keeps cost supplemental: if discovery or fetch fails, the session falls back to normal resource rendering

What changed

• src/config.rs: loads ~/.config/kuberift/config.toml
• src/cost.rs: service discovery, allocation parsing, pod-owner/controller aggregation, formatting
• src/items.rs: optional cost column + expensive-resource highlighting
• src/k8s/resources.rs: applies a pre-fetched cost snapshot during initial render and live updates
• README.md: documents config and runtime behavior

Notes

• direct endpoint override works via cost.opencost_endpoint
• auto-discovery prefers opencost / kubecost services and uses the Kubernetes service proxy path
• the cost path is bounded by a short session prefetch timeout; if it misses that window, cost is disabled for the session instead of holding the UI open

Validation

• cargo test