Add configurable default blog author name

[CySSoO]

Motivation

• Provide a configurable fallback author name to show on posts that have no linked/active author instead of always using the shop name.
• Allow store admins to set a default author display name in the module configuration for better branding and localization.

Description

• Introduce a new configuration key EVERBLOG_DEFAULT_AUTHOR_NAME and initialize it on install to Configuration::get('PS_SHOP_NAME').
• Expose the new setting in the module configuration form by adding a text field EVERBLOG_DEFAULT_AUTHOR_NAME and include it in getConfigFormValues().
• Validate the submitted value in postValidation() using Validate::isGenericName() and persist it in postProcess() like other settings.
• Use the configured fallback in controllers/front/post.php when a post has no linked/active author, falling back to PS_SHOP_NAME if the setting is empty.

Testing

• Ran php -l everpsblog.php and it reported no syntax errors (success).
• Ran php -l controllers/front/post.php and it reported no syntax errors (success).
• Attempted an automated Playwright screenshot of a running instance which failed because no server was listening at 127.0.0.1:8080 (failure).

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Codex Task