Fix default port to match Dockerfile (8080 -> 8000)

[ddulic]

Summary

• The Dockerfile sets SUPERNOTE_PORT=8000 but config.py still defaulted to 8080, causing a mismatch when running outside Docker
• Updates the ServerConfig.port default and the base_url docstring comment to reflect 8000

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
supernote/cli/server.py	0.00%	1 Missing ⚠️

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