Add Kubernetes support alongside Docker

[TheKnarf]

Summary

This PR adds support for running Dave in Kubernetes clusters by reading service annotations instead of Docker container labels.

• Add src/providers/ with unified provider interface
• Add src/kubernetes/ for Kubernetes API integration
• Update src/pages/index.tsx to use the new provider abstraction
• Update README with Kubernetes deployment instructions

How it works

The provider is auto-detected based on environment:

• In Kubernetes: Detects KUBERNETES_SERVICE_HOST env var
• In Docker: Checks for /var/run/docker.sock
• Can be overridden with DAVE_PROVIDER env var (docker, kubernetes, or auto)

Kubernetes Usage

Services can use the same annotation format as Docker labels:

apiVersion: v1
kind: Service
metadata:
  name: my-app
  annotations:
    dave.name: "My Application"
    dave.url: "http://my-app.example.com"
    dave.icon: "mdi:application"

Test plan

• Test Docker provider still works (existing functionality)
• Test Kubernetes provider in a k3s/k8s cluster
• Verify auto-detection works correctly

Notes

The Kubernetes provider uses native https module to call the Kubernetes API, avoiding heavy dependencies like @kubernetes/client-node that have Node version requirements.

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