feat(web): drop the empty terms checkbox from registration

The register step asked the user to accept "the terms of use" with no terms
linked anywhere — ceremony accepting nothing on a friends-only tool (UX review
C4). Remove the checkbox and the server-side acceptance requirement; only a
display name is required now.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/register_test.go

@@ -46,7 +46,7 @@ func TestRegisterCreatesExactlyOneUserAndIdentity(t *testing.T) {
 	truncateAll(t, p)
 
 	req := httptest.NewRequest(http.MethodPost, "/register",
-		strings.NewReader("display_name=Newbie&accept_terms=yes"))
+		strings.NewReader("display_name=Newbie"))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	rec := do(t, app, req)
 	require.Equal(t, http.StatusSeeOther, rec.Code)
@@ -122,7 +122,7 @@ func TestRegisterRejectsMissingFields(t *testing.T) {
 	truncateAll(t, rawPool(t))
 
 	req := httptest.NewRequest(http.MethodPost, "/register",
-		strings.NewReader("display_name=&accept_terms=")) // both missing
+		strings.NewReader("display_name=")) // missing display name
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	rec := do(t, app, req)
 	require.Equal(t, http.StatusBadRequest, rec.Code)