fix: validate CLI port argument

[WilliamK112]

Summary

• validate --port as an integer before starting the dashboard
• reject non-numeric, zero/negative, fractional, and greater-than-65535 values with a clear CLI error
• preserve the default port and valid boundary values

Closes #37.

Validation

• npm run build
• npm test — 8 test files, 120 tests passed
• npx tsx src/cli.ts --port 0 exits 1 with the expected error
• npx tsx src/cli.ts --port -1 exits 1 with the expected error
• npx tsx src/cli.ts --port 99999 exits 1 with the expected error
• npx tsx src/cli.ts --port abc exits 1 with the expected error
• npx tsx src/cli.ts --port 3000.5 exits 1 with the expected error
• npx tsx src/cli.ts --port 65535 starts successfully
• git diff --check

Revalidated after refreshing the branch onto current main with Node.js 24.14.0.

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[WilliamK112]

Refreshed this onto current main (head d35506d) and revalidated it with Node.js 24.14.0. npm run build passes, all 120 existing tests pass, invalid/non-integral ports exit with the expected message, and port 65535 starts successfully. This PR remains limited to the assigned CLI port-validation issue #37.

[WilliamK112]

Scope note: while checking the related issues, I found that #90 and #117 are already covered by the earlier contribution #144. I removed my overlapping test commit and the closing claims for those issues so that #144 retains its scope and attribution. This PR is back to the small, assigned #37 change only. Because #37 and #90 describe the same underlying validation, I will leave the preferred merge path to the maintainers and will not add further overlapping scope here.