feat(admin): visible Test/Test-seed badges + filter toggle

Users table shows [Test] next to soak-harness accounts, invite codes
list shows [Test-seed] next to codes that flag new accounts as test,
and a new "Include test accounts" checkbox lets admins hide bot
traffic from the user list.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/admin.html

@@ -114,6 +114,10 @@
                         <input type="checkbox" id="include-banned" checked>
                         Include banned
                     </label>
+                    <label class="checkbox-label">
+                        <input type="checkbox" id="include-test" checked>
+                        Include test accounts
+                    </label>
                 </div>
                 <table id="users-table" class="data-table">
                     <thead>