Add Quick Start onboarding wizard for first-time PPOM setup

Problem

Evaluators report "Clearer instructions," "I cannot do it," "it should be a simple task." The admin UX requires navigating to a separate PPOM menu, creating a field group, then separately assigning it — these steps aren't connected. Quick-churn users (<1hr) never figure this out.

Solution

New PPOM_Onboarding class that hooks into ppom_before_field_groups_list:

1. Detects first visit (no non-demo field groups in nm_personalized table)
2. Shows inline 3-step wizard: choose field type, name the field, assign to product
3. AJAX handler creates the field group and assigns it
4. "Skip" button dismisses permanently via ppom_onboarding_dismissed option

Files Affected

• New: classes/onboarding.class.php — wizard class
• classes/admin.class.php — add ppom_before_field_groups_list action hook
• woocommerce-product-addon.php — require + instantiate

Acceptance Criteria

• Wizard appears on first visit to PPOM admin (no field groups)
• 3-step flow creates a working field group assigned to a product
• Wizard disappears after first group is created
• "Skip" button permanently dismisses
• AJAX nonce and capability checks present

Priority: Medium — reduces evaluator churn
Regression risk: Isolated — new admin UI

Start by reading classes/admin.class.php and woocommerce-product-addon.php to understand the existing field-group list hook and loading path, then inspect the new classes/onboarding.class.php entry point. Done means the first-visit wizard completes all three steps, creates and assigns a field group, disappears afterward, and supports permanent dismissal with nonce and capability checks.