Issue #391: Introduce swh_get_option() helper as read-through wrapper (de-risks v4.0 schema split)

Labels: enhancement, dx Milestone: v3.7.0 — v4 Foundations

Body

v4.0’s #356 splits monolithic `swh_options` into 5 sub-options. This touches every admin page and every read site in the plugin — the riskiest single change in v4.0.

Land the helper signature now (still backed by the monolithic option) so v4.0’s migration only changes the implementation, not the call sites.

Plan

Add `swh_get_option( string $group, string $key, mixed $default = null ): mixed` to `includes/helpers.php`. v3.7 implementation:

```php function swh_get_option( $group, $key, $default = null ) { $opts = get_option( ‘swh_options’, array() ); return isset( $opts[ $key ] ) ? $opts[ $key ] : $default; } ```

The `$group` arg is currently ignored but baked into the signature. v4.0 changes the body to read from the correct sub-option per group; call sites stay identical.

Acceptance

• Helper added with full signature and PHPStan typing
• All `get_option(‘swh_options’)[…]` reads in `admin/`, `includes/`, `frontend/` migrated to `swh_get_option()` (grep-verified)
• PHPUnit: default fallback, missing key, missing option entirely
• PHPStan level 9 clean
• Playwright: full settings round-trip across all 8 tabs still passes
• CHANGELOG: "Added `swh_get_option()` helper (preparing for v4.0 schema split)."

Dependencies

Unblocks risk on v4.0 #356 (settings schema migration).