STRWEB-156 Gracefully handle a missing favicon

[alb3rtino]

https://folio-org.atlassian.net/browse/STRWEB-156

Purpose

Running yarn start (i.e. stripes serve) in a module repository without a stripes.config.js fails during webpack configuration. The default config does not define branding.favicon.src, and the fallback FAVICON_PATH is resolved relative to the current working directory, where the asset typically does not exist. As a result, developers have to manually create a stripes.config.js or place a favicon at ./tenant-assets/folio-favicon.png to start a module locally.

#184 worked around this for the test environment by suppressing the missing-favicon error when NODE_ENV === 'test', but the underlying issue remains for yarn start. This PR removes that special case and resolves the favicon uniformly across environments.

Approach

• Introduced resolveFaviconPath helper that returns the configured favicon if it exists, the default FAVICON_PATH if it exists, or the bundled default-assets/favicon.svg otherwise.
• A configured branding.favicon.src that does not exist still throws, so a typo in a tenant config remains a hard error.
• Removed the now unused isTesting flag.
• Updated CHANGELOG.md.

[alb3rtino]

Moved to #188