fix: use add_filter() for plugin_action_links hook

[thisismyurl]

What

plugin_action_links_{$plugin} is a filter, not an action — callbacks
must return the modified $links array. The hook in Plugin::run() was
registered with add_action(), which silently discards the return value
of add_plugin_action_links(), so the Settings link was never injected
into the Plugins list table.

Before:

add_action( 'plugin_action_links_' . plugin_basename( $this->path ) . '/login-with-google.php', [ $this, 'add_plugin_action_links' ] );

After:

add_filter( 'plugin_action_links_' . plugin_basename( $this->path ) . '/login-with-google.php', [ $this, 'add_plugin_action_links' ] );

Why add_filter()?

add_action() passes null as the first argument to the callback and
ignores the return value. add_filter() passes the existing $links array
and uses the returned value to replace it — which is exactly what
add_plugin_action_links() relies on when calling array_merge().

Testing

1. Install the plugin on a fresh WordPress site.
2. Go to Plugins → Installed Plugins.
3. Confirm a Settings link appears under "Login With Google."

Closes #331

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[Copilot]

Pull request overview

Note

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Adjusts WordPress hook registration to use the correct API for modifying plugin action links.

Changes:

• Switches from add_action to add_filter for the plugin_action_links_{plugin} hook.

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