Newsletter: rename the Access panel to Audience and explain post access (#51262)

* Newsletter: rename the Access panel to Audience and explain post access

The Access panel did not tell creators that it controls who can read the
post, and it offered "Paid subscribers only" with no indication of how to
turn paid subscriptions on.

- Rename the panel to Audience in both the sidebar and pre-publish views,
  and give the radio group the visible legend "Who can read this post?".
- Shorten the options to "Subscribers" and "Paid subscribers".
- Describe the selected level below the group. The description accounts for
  paywall blocks, where a paid post is still emailed to every subscriber
  because free subscribers receive the portion above the paywall.
- When Stripe is not connected or no tier exists, show "Paid subscribers"
  disabled with a link to set it up rather than opening the setup dialog.
  The option stays selectable when it is already the saved value, so a post
  set to paid before Stripe was disconnected keeps its selection.
- Point getPaidPlanLink at Jetpack Cloud on self-hosted sites, which serve
  these screens under /monetize rather than /earn.

Fixes NL-766
Fixes NL-794

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Newsletter: use the shared Link component for the paid setup link

The editor already has a design-system external link — @wordpress/ui's Link
with openInNewTab — used by the SEO panel and a dozen other extensions. It
renders the underline, the diagonal arrow, and an "(opens in a new tab)"
label, so the hand-rolled underline and @wordpress/icons external glyph are
no longer needed. This also matches the arrow in the design.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Newsletter: keep the access descriptions extractable in production builds

The minifier merged each pair of __() calls in getAccessDescription into a
single call with a ternary msgid, which fails the i18n check. Development
builds do not minify, so this only surfaced in the production build. Adding
an unused third argument to one branch keeps the calls distinct.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Newsletter: say "all subscribers" for a paid post with a paywall block

A paid post containing a paywall block is emailed to every subscriber, not
only paid ones, because free subscribers receive the portion above the
paywall. "Subscribers receive it by email" read as though the paid audience
was meant; spell out that it goes to all of them, matching the wording
getAccessLabelForCopy already uses for the same case.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Newsletter: keep the unavailable paid option reachable by keyboard

A native disabled attribute took the paid option out of the tab order and
hid it from screen readers, which defeats the point of showing it: the
whole reason it stays visible is so creators discover paid newsletters are
available. Mark it aria-disabled instead, so it is announced and focusable
but still cannot be selected, and point aria-describedby at the setup link
so assistive technology reads the remedy along with the option.

Also record why the paid count does not follow postHasPaywallBlock. The
count is the size of the audience that can read the level; forwarding the
flag would switch it to the email reach and make both options report the
same total on a paywalled post. Who receives the email is stated in the
description instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Newsletter: match the pre-publish heading to the subscribers option label

Shortening the option labels aligned the pre-publish panel heading with the
radio label for Everyone and Paid subscribers, leaving Subscribers as the
only option whose heading differed from what the creator selected. The
heading exists so the choice can be checked while the panel is collapsed,
so a different word there means expanding the panel to confirm.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/31812335476

Upstream-Ref: Automattic/jetpack@e88fe01

Author: LiamSarsfield