invoices: fix 500 on the admin invoice page

Replacing the paid button with the cancel/hide action block cut the old
markup at the first <% end %>, which belonged to the button's inner
if/else, leaving a stray </button></form><% end %> behind. show.erb no
longer compiled and every admin invoice page returned 500; nothing
rendered that page in tests.

Remove the leftovers, render the page in every state (pending, paid,
cancelled, hidden), and compile every view in a test so an unbalanced
template fails CI even when no test visits that page.

Assisted-by: Claude:opus-5

views/admin/invoices/show.erb

@@ -68,9 +68,6 @@
         </form>
         <% end %>
     </div>
-    <% end %>
-        </button>
-    </form>
     <% end %>
     <p style="margin-top: 14px;">
         <% if can?('invoices', 'write') %><a href="/admin/invoices/<%= @invoice.uuid %>/duplicate">Use as template for a new invoice →</a><% end %>