Predefined config at first load

[spwoodcock]

• I love the current setup where the user can input any STAC URL and browse as needed.
• For deploying stac-map alongside an existing STAC, it would be neat if we can define a configuration somewhere to automatically load a specific STAC.
• Option 1: defined at build as .env or similar. Would mean user has to rebuild.
• Option 2: defined dynamically as say a config.json file that is loaded on first mount. If /config.json is present, load it, else fallback to the default of no pre-defined config.
• I'm sure there are other options too!