Add proactive compatibility warnings for known conflicting plugins

Problem

Users report plugin conflicts without knowing which plugin causes the issue. PPOM has documented conflicts with Checkout Field Editor (#472), Woo Discount Rules (#426), and WooCommerce Product Bundles, but doesn't warn users proactively.

Solution

Add ppom_compatibility_check_notices() — checks get_option('active_plugins') against a known conflicts array. Shows dismissible admin notice with plugin name, issue description, and docs link.

Known conflicts to detect:

1. Checkout Field Editor and Manager for WooCommerce — may duplicate/override PPOM fields at checkout
2. Woo Discount Rules — may miscalculate totals when addon prices are active
3. WooCommerce Product Bundles — PPOM fields may not render inside bundled products

Files Affected

• inc/admin.php — new ppom_compatibility_check_notices() function
• classes/plugin.class.php — hook registration

Acceptance Criteria

• Notice appears when a known conflicting plugin is active
• Notice links to relevant documentation
• Notice is dismissible
• No performance impact (checks cached active_plugins option)

Priority: Medium
Regression risk: Isolated — admin-only

Related: #472, #426

Start in inc/admin.php and inspect classes/plugin.class.php to see how admin hooks are registered. Add the compatibility notice flow for the three listed plugins, then verify that active plugins trigger dismissible notices with issue descriptions and documentation links without extra active-plugin lookups.