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Changes proposed in this Pull Request:

Adds end-to-end group subscription self-service in My Account. Owners can manage members and invites from a dedicated group page; members can leave a group; multi-group owners get a picker. Publishers can override the "group" terminology (singular + plural) per site.

Owner-facing

• New /my-account/group/ endpoint. Single-group owners drop straight into management; multi-group owners see a card picker showing member count and subscription status.
• Empty-state UI (centered card, stacked invite actions) when a group has no members and no invites.
• Populated state uses a tabbed view (Members / Invitations) with per-row dropdown actions.
• Remove member modal with member-specific copy and an "Are you sure?" confirmation heading.
• Disable invite link modal now includes the "Are you sure?" heading, matching the pattern used by Leave group, Remove member, Delete account, and Cancel subscription.
• Invitation row actions disambiguated: "Resend" → "Resend invite", "Cancel" → "Cancel invite".
• Invite submit button shows a loading spinner during navigation.
• Header "Invite members" dropdown only renders when the group has members or invites (suppressed in empty state).
• Per-member join timestamps recorded and surfaced as "Member since" on the subscription details view.

Member-facing

• Leave group flow on the subscription header, with a destructive confirm modal and loss-aversion copy.
• Owner contact privacy: the subscription header shows the owner's display name only, not their email.
• Orders and Payment information tabs are hidden for group members who have no orders of their own, removing empty-state noise from a member's My Account.

Newspack UI library

• New tabs component (used by Members/Invitations view) with aria-selected / tabindex keyboard handling.
• New __grid primitive — real CSS grid (was responsive flex on the old __row). Default 1 col mobile / 2 cols desktop. Modifiers: --cols-{3,4,5,6} for fixed column counts, --gap-{0..12} for the gap scale, and --subgrid for shared column tracks across child rows.
• Box utility additions: --2x-large size modifier, :has(> __box__actions) semantic action layout, outline-button hover for anchor variants.
• Dropdown rewritten with document-level event delegation so dropdowns inside tab panels (which the tabs controller restores on switch) keep working. Opening a dropdown now closes any other open one. New --align-start/center/end modifiers.
• Modal form-submit handler now also catches nested <form> inside <section class="newspack-ui__modal__content">, not only the top-level case. Submit button labels wrapped in <span> so the loading spinner CSS works universally. Fetch-backed actions add the loading class to the button, not the span (prevents duplicate submits).
• WC notice → Newspack snackbar conversion now handles WC Blocks markup (.wc-block-components-notice-banner.is-success) in addition to legacy .woocommerce-message.

Delete account modal

• Modal sizes itself medium when the user has subscriptions / donations / newsletters to suggest, small otherwise.
• Alternative-action rows (Subscriptions, Newsletters) refactored from the old __row + __width--40 layout into a __stack --horizontal per row, wrapped in a __grid --subgrid container so the action buttons align to the same column width regardless of label length.

Publisher controls (Audience → Setup)

• "Group labels" override: publisher can set singular + plural terms (e.g. Team / Teams) that propagate through all reader-facing strings via Group_Subscription::get_label().

Utilities

• newspack_get_user_display_label() in includes/util.php: strategy-driven cascade for resolving a user's display name.
• New Group_Subscription_MyAccount::is_subscription_manageable() helper, used to derive the read-only state for cancelled/expired group subscriptions.

How to test the changes in this Pull Request:

Test using the QA seed at html/qa-seed-group-subs.php (run wp eval-file /var/www/html/qa-seed-group-subs.php). Password for every seeded user is password.

Owner — multi-group picker

1. Log in as marcus.aldridge@example.com and visit /my-account/. Confirm a sidebar entry exists for the group endpoint.
2. Click into it. With no subscription ID, the picker appears with two cards (one per group), each showing member count + status badge.
3. Click a card. Lands on /my-account/group/{sub_id}/.

Owner — empty state

1. Still as Marcus, clear members and invites from one of his groups (the family group), then visit that group's page.
2. Confirm the centered empty-state box renders with three stacked actions (Invite by email, Copy invite link, Disable invite link is hidden until a link invite exists).
3. The header "Invite members" dropdown is NOT shown in the empty state.

Owner — populated state

1. Visit a populated group (e.g. Marcus's "Aldridge & Partners LLP", which the seed creates with custom name + 2 members + 1 invite). The tabs render with Members (3) / Invitations (1).
2. Open the per-row dropdown on a non-manager member → click "Remove member". Modal opens with "Are you sure?" + the member's name. Confirm cancels.
3. Switch to the Invitations tab. Open the row dropdown → see "Resend invite" / "Cancel invite" (was "Resend" / "Cancel"). Confirm "Cancel invite" submits directly without a modal.
4. Open the header "Invite members" dropdown → "Disable invite link" → modal shows the header "Disable invite link" AND the body "Are you sure?" heading.

Owner — invite flow spinner

1. From the empty state OR the header dropdown, click "Invite by email".
2. Enter an email and submit. The submit button label disappears and a spinner animates in its place while the page navigates.
3. After redirect, the WC notice is converted into a Newspack snackbar.

Member — leave group

1. Log in as layla.shah@example.com (member of Imran's group).
2. Visit /my-account/view-subscription/{imran_sub_id}/. Confirm the subscription header shows Imran's display name only (no email).
3. Click "Leave group" → modal opens with "Are you sure?" + loss-aversion copy. Cancel closes the modal.

Member — hidden tabs

1. Still as Layla, visit /my-account/. Navigation should NOT include Orders or Payment information (she's a group member with no orders of her own).
2. Log in as a regular reader with orders (any seeded user with their own subscription) → both tabs appear.

Cancelled subscription owner

1. Log in as hassan.khoury@example.com (cancelled group). Visit his group page.
2. The "This group is no longer active…" info notice appears at the top, and management actions are suppressed.

Publisher label overrides

1. As admin, visit Audience → Setup → Group labels.
2. Override singular to "Team" + plural to "Teams". Save.
3. Reload an owner's group page. All reader-facing strings now use "team"/"Teams" (button labels, modal copy, picker heading, empty-state copy, snackbar messages). The URL slug remains /group/ (this is intentional — see the discussion thread).

Member since timestamps

1. As Marcus, manage a populated group. Each member row's tooltip/detail surfaces a "Member since" date relative to when they joined.
2. Add a new member via invite → join timestamp is written when the invite is accepted.

Delete account modal

1. Log in as eleanor.whitfield@example.com (she owns 1 active subscription).
2. My Account → Account details → "Delete my account" → modal opens at medium size.
3. Confirm: "Are you sure?" heading, body copy, then one or more alternative-action rows. The "Manage X" buttons align to the same column width across rows (subgrid).
4. Log in as a reader with no subscriptions / donations / newsletters → same modal opens at small size with no alternative rows.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?