feat(admin): admin UX modernisation — settings rebuild + shared scaffolding (NEWS-2152, NEWS-2168, NEWS-2263)

[thomasguillot]

Companion to Automattic/newspack-newsletters#2108 (NEWS-2152) and Automattic/newspack-newsletters#2110 (NEWS-2168).

Started as the wizard-bridge wiring for <SubscriptionLists> (NEWS-2152 / NEWS-2168) and grew to absorb the broader Newsletters Settings page rebuild and the shared scaffolding it needed (NEWS-2263 / admin UX modernisation milestone). Branch renamed from news-2152-bundled-mode-modal-parity to news-admin-ux-modernisation to reflect the broader scope.

NEWS-2152 / NEWS-2168 — Subscription-list bridge + ESP-edit parity

• <SubscriptionLists> per-row Edit / Delete buttons dispatch wizard-bridge events instead of opening legacy URLs; reloads on save / delete.
• Edit on remote rows goes through the bridge with kind: 'esp' (no audience picker, title + description only).
• Active toggle commits per-row via PATCH /lists/{db_id}; bulk Save button retired.
• 500 ms fallback timer routes to the legacy CPT editor when the bridge bundle is missing.

NEWS-2263 — Newsletters Settings page rebuild

Single-section wizard (fixedHeader, dirty tracking, unsaved-changes guard) with sections, top-to-bottom:

• Email service provider — 2x2 brand-icon card grid (Active Campaign, Mailchimp, Constant Contact, Manual / Other). Selecting a card sets the provider and reveals its credentials below the grid. OAuth Authorize prompt inlines for Constant Contact.
• Newsletter posts — slug, comments, related-posts as ToggleControls.
• Subscription lists — per-row ToggleControl with Local / ESP badge, list description as help text, divider between rows, Edit / Delete on the right (Delete only for local lists). Below the list, a paragraph + secondary "Add new local list" button explain provider-specific syncing (label resolved per-ESP via the wizard's settings response).
• Ads tracking — merged in from the previous separate tab; ToggleControls for click + impressions. Tracking tab removed from the ads-list admin header since the section now lives in Settings.
• Letterhead — separate section at the bottom for the Letterhead API key + help link.

PHP: wizard /settings response now includes the active provider's labels (get_labels('local_list_explanation')), guarded with class_exists for standalone-newspack-plugin installs.

Shared scaffolding (packages/components)

• integration-icons namespace with 6 brand SVGs (Active Campaign, Constant Contact, Fundraise Up, Mailchimp, Salesforce, Wisepops); fills customisable via var(--integration-icon-color) / --integration-icon-color-accent with brand-colour defaults.
• useUnsavedChangesDialog hook — wraps useConfirmDialog with the standard "discard changes" messaging, intercepts outbound link clicks so the custom dialog fires instead of the silent native one, plus a beforeunload safety net.
• CardSettingsGroup gains optional className and iconSize props.
• CoreCard accepts iconSize; SCSS reads it via the new --newspack-card-icon-size custom property with fallback to existing defaults (no consumer changes required).
• SectionHeader — container margin now zeroes alongside its inner element when noMargin is set (via :has() so callsites need no change).
• admin.css — chrome section header only gets margin-top when it's not the first child of newspack-wizard__content, fixing the doubled spacing on audience /edit/new/all.
• Audience content-gates edit adopts useUnsavedChangesDialog so the same standardised confirm flow covers both wizards.

Test plan

• Run paired newspack-newsletters branches (NEWS-2152 already merged into the epic; NEWS-2168 on news-2168-revisit-esp-subscription-list-edit-flow-standalone).
• Smoke-test Add / Edit / Delete on a Mailchimp + Constant Contact dev site for both local and remote rows.
• Verify the bottom Save Subscription Lists button is gone and the active toggle on a remote row persists across reloads.
• Disable newspack-newsletters bundle JS to confirm the fallback timer redirects to the legacy URL on Edit.
• Newsletters Settings: pick each ESP card, verify its credential fields appear; toggle settings; click Save; confirm the unsaved-changes guard fires on navigation away with a dirty form.
• Audience access control /edit/new/all: confirm the chrome section header sits flush with the wizard content padding (no doubled top margin) and the discard-changes dialog still fires via the shared hook.

Merge order

Merge the newspack-newsletters branches first; this PR follows.

[thomasguillot]

Branch was renamed from news-2152-bundled-mode-modal-parity to news-admin-ux-modernisation to reflect the broader scope. GitHub's rename endpoint didn't transitively update this PR's head_ref so it auto-closed. Continued in #4735.