Fixed issue 420 Use as much screen estate as possible on initial load

[kehach07]

Checklist

I have read the OpenWISP Contributing Guidelines
.I have manually tested the changes proposed in this pull request.
I have written new test cases for new code and/or updated existing tests for changes to existing code.
I have updated the documentation.

Reference to Existing Issue
Closes #420.

Description of Changes
Improves the initial rendering behavior by adapting the initial zoom level to the graph size.

Small graphs are automatically zoomed in so node labels are visible, while large graphs keep the default zoom to use available screen estate and avoid label clutter. This works consistently with the label visibility logic introduced in #419.

[nemesifier]

I'm looking for a solution to embed in the library with at least 1 automated test, this solutions instead implements the logic in the example, which is not reusable and doesn't add any test, therefore cannot be accepted as is. Feel free to the OpenWISP dev chat to discuss the details of this.