View a policy version's content, and make policy text readable

The Policies admin page listed versions but never showed what any of them
said, so revising a policy meant retyping it blind into an empty draft box.
Each version row now has a View action that renders that version's text on
the page, editable in place. A draft is saved back to itself; editing a
published or archived version branches a new draft and leaves the original
alone, because acceptances are recorded against policy_version_id and text a
student agreed to must stay exactly as they saw it.

That viewer also exposed why a studio reported the acceptance box as
unreadable — one squashed line, overlapping words, a horizontal scrollbar.
Bodies are typed into a bare textarea, so most carry no markup, and the raw
text was emitted with its blank lines intact but nothing to turn them into
paragraphs. PolicyVersion::bodyHtml() now renders every body the way
WordPress renders post content (kses, then wpautop) and feeds all three
consumers: the booking/enrolment JSON, the signup form, and the new viewer.
Bodies written with markup are unaffected.

The other half was that .us-policy-body had no CSS whatsoever and inherited
whatever the theme did with an unstyled block in a form. It is now a bounded
reading box that scrolls vertically and breaks long tokens, so a pasted URL
cannot force the page sideways and a long policy cannot push the accept
checkbox out of view. RegistrationPage was also never enqueueing the plugin
stylesheet, which is why the signup gate looked worst of all.

Closes #126
Closes #127

Co-Authored-By: Claude Opus 5 <[email protected]>

tests/Unit/Auth/RegistrationPageTest.php

@@ -37,6 +37,8 @@ class RegistrationPageTest extends TestCase
         Functions\when('sanitize_email')->alias(static fn ($v) => $v);
         Functions\when('absint')->alias(static fn ($v) => (int) $v);
         Functions\when('current_time')->justReturn('2024-01-01 00:00:00');
+        Functions\when('wp_enqueue_style')->justReturn(null);
+        Functions\when('wp_enqueue_script')->justReturn(null);
 
         $invites   = Mockery::mock(InviteRepository::class);
         $policies  = Mockery::mock(PolicyRepository::class);