refactor: simplify toggle handlers to return 204 No Content

Remove empty toggle templates (length-toggle.html, theme-toggle.html,
logo-toggle.html) that were just placeholders. The frontend uses
hx-swap="none" so the response body was always ignored anyway.

Now the handlers:
- Set the preference cookie
- Return 204 No Content immediately
- Hyperscript handles the UI state toggle on the frontend

This removes unnecessary template rendering overhead and cleans up
dead code. Tests updated to expect 204 instead of 200.

internal/handlers/cv_htmx_test.go

@@ -60,8 +60,9 @@ func TestToggleLength(t *testing.T) {
 			w := httptest.NewRecorder()
 			handler.ToggleLength(w, req)
 
-			if w.Code != http.StatusOK {
-				t.Errorf("Expected status OK, got %d", w.Code)
+			// 204 No Content - frontend uses hx-swap="none" so response body is ignored
+			if w.Code != http.StatusNoContent {
+				t.Errorf("Expected status No Content (204), got %d", w.Code)
 			}
 
 			// Check that response sets the toggled cookie
@@ -131,8 +132,9 @@ func TestToggleIcons(t *testing.T) {
 			w := httptest.NewRecorder()
 			handler.ToggleIcons(w, req)
 
-			if w.Code != http.StatusOK {
-				t.Errorf("Expected status OK, got %d", w.Code)
+			// 204 No Content - frontend uses hx-swap="none" so response body is ignored
+			if w.Code != http.StatusNoContent {
+				t.Errorf("Expected status No Content (204), got %d", w.Code)
 			}
 		})
 	}
@@ -253,8 +255,9 @@ func TestToggleTheme(t *testing.T) {
 			w := httptest.NewRecorder()
 			handler.ToggleTheme(w, req)
 
-			if w.Code != http.StatusOK {
-				t.Errorf("Expected status OK, got %d", w.Code)
+			// 204 No Content - frontend uses hx-swap="none" so response body is ignored
+			if w.Code != http.StatusNoContent {
+				t.Errorf("Expected status No Content (204), got %d", w.Code)
 			}
 
 			// Verify theme cookie was set