JOIN-150 Add optional income guidance to the membership step - #125

Why

GMTU members said seeing suggested dues against their income helps them choose a tier, as on CATU's join page. This adds an optional table of income bands to the membership step. It is off by default and an add-on switches it on, so other sites see no change.

How

• New filter ck_join_flow_income_guidance_enabled, false by default. GMTU's add-on returns true (common-knowledge-join-flow-gmtu#13).
• With it on, each membership plan (global or a block's own) gets optional Weekly income and Monthly income text fields. The Copy tab gets an editable Income guidance heading and an optional Income guidance note.
• The membership step shows a table of the banded plans above the tiers. Choosing a row selects that tier, and choosing a tier marks its row. Plans with no band are left out. On phones each band becomes a labelled card.
• The note is passed through wp_kses_post(), so an admin without unfiltered_html can't use it to run script.
• Version 1.4.45.

Test plan

• With no add-on switching it on, the settings page and membership step look exactly as before
• Switch it on (a must-use plugin with add_filter('ck_join_flow_income_guidance_enabled', '__return_true');)
• Weekly income and Monthly income appear on each global plan, and on a block's own plans
• The Copy tab shows the heading, with its default, and the note
• Give some plans bands and save; the table shows them with the heading, and the note when set
• Clicking a row selects its tier, and the button shows that tier's amount
• Choosing a tier directly highlights its row
• Continue reaches payment with the chosen tier
• On a phone, each band shows as its own card

Automated: PHP unit tests, Jest, and e2e specs for the settings, the table and the flag being off. CI runs all of them, plus Prettier, phpcs and PHPStan.

Watch out for

• Merge this before releasing GMTU#13, which needs 1.4.45.
• E2E setup: a must-use plugin in packages/join-e2e/mu-plugins/ switches the flag on, mounted in docker-compose.yml. The admin helpers now live in tests/helpers.ts, and the plan redirect spec uses them too.
• The block editor income test picks its page from the full list, not the picker's search, because of JOIN-158.
• The same unsanitised HTML exists in ten older copy settings on master. That's a separate fix, not this PR.

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