Add arkade-explorer to the Ark service stack

[Kukks]

Summary

• Adds the arkade-explorer container (ghcr.io/arklabshq/arkade-explorer:latest) to the docker-compose, started automatically when --ark is enabled
• Explorer available at http://localhost:7080
• Patches the Vite-baked API URLs at container startup to point at the local arkd (http://localhost:7070)
• Bumps arkd and ark-wallet images to v0.9.0-rc.1 and nbxplorer to 2.6.0

Test plan

• Run nigiri start --ark and verify ark-explorer container is running
• Open http://localhost:7080 and confirm the explorer loads
• Verify the explorer connects to the local arkd at localhost:7070
• Run nigiri start (without --ark) and verify ark-explorer is NOT started

[Kukks]

Closing as obsolete — every change in this PR has already landed on master:

• The ark-explorer service block in cmd/nigiri/resources/docker-compose.yml (identical image, same sed patch, same 7080:80 port mapping)
• The "ark-explorer" append to the services list in cmd/nigiri/start.go when --ark is set
• arkd / arkd-wallet are now at v0.9.3 (with Update ark-wallet and ark images to v0.9.4 #259 bumping to v0.9.4), well past the v0.9.0-rc.1 proposed here
• nbxplorer is already at 2.6.0

Re-merging would be a no-op (or a downgrade for the image versions).