fix: Resolve CSS bundling in production and lint errors

- Fix golangci-lint errcheck errors by using t.Setenv() instead of os.Setenv()
- Add CSS bundle build step to deploy workflow for production
- Add graceful fallback to modular CSS if bundle doesn't exist
- Remove unused os import from preferences_test.go

internal/middleware/preferences_test.go

@@ -3,7 +3,6 @@ package middleware
 import (
 	"net/http"
 	"net/http/httptest"
-	"os"
 	"testing"
 )
 
@@ -385,12 +384,8 @@ func TestIsProductionMode(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			// Save original environment
-			originalEnv := os.Getenv("GO_ENV")
-			defer os.Setenv("GO_ENV", originalEnv)
-
-			// Set test environment
-			os.Setenv("GO_ENV", tt.envValue)
+			// t.Setenv automatically restores the original value after the test
+			t.Setenv("GO_ENV", tt.envValue)
 
 			result := isProductionMode()
 
@@ -428,12 +423,8 @@ func TestSetPreferenceCookieSecureFlag(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			// Save original environment
-			originalEnv := os.Getenv("GO_ENV")
-			defer os.Setenv("GO_ENV", originalEnv)
-
-			// Set test environment
-			os.Setenv("GO_ENV", tt.envValue)
+			// t.Setenv automatically restores the original value after the test
+			t.Setenv("GO_ENV", tt.envValue)
 
 			w := httptest.NewRecorder()
 			SetPreferenceCookie(w, "test-cookie", "test-value")