Say "student" and "profile" in the UI, not "child" and "family"

Sweep the translatable strings across the frontend templates, the admin
screens, the editor previews and the block inserter entry. Nothing else
moves: the database columns, request parameters, form field names, CSS
classes, the us_family shortcode and the us-scheduler/family block name are
contracts with existing installs and with post content people have already
saved, so renaming them would break sites for no user-visible gain.

Two strings are reworded rather than swapped, because the direct
substitution reads wrong:

- The students list said "Child of Jane" and now says "Managed by Jane".
  "Student of Jane" would read as a teacher's pupil, which is exactly the
  wrong idea in a music studio.
- A managed account is now "a managed student account" rather than "a
  student account", which would not distinguish it from the account holder.

The guardian feature doc gains a short section on the split, so the next
person to work on it does not read the mismatch as drift and "fix" it.

Closes #144

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

tests/Unit/Auth/RegistrationPageTest.php

@@ -667,7 +667,7 @@ class RegistrationPageTest extends TestCase
 
         $result = $this->submit(new Invite(email: '[email protected]', token: 'hash'), false);
 
-        self::assertStringContainsString('at least one child', $result);
+        self::assertStringContainsString('at least one student', $result);
     }
 
     /**
@@ -694,7 +694,7 @@ class RegistrationPageTest extends TestCase
         Functions\expect('wp_insert_user')->never();
         $this->ctx['guardians']->shouldNotReceive('createChild');
 
-        self::assertStringContainsString('for each child', $this->submit(new Invite(email: '[email protected]', token: 'hash'), false));
+        self::assertStringContainsString('for each student', $this->submit(new Invite(email: '[email protected]', token: 'hash'), false));
     }
 
     /**