Move all theme environment variable logic to a utils function

Move theme environment variable logic to a utils function and update routes to use it.

* Add `GetThemeFromEnv` function in `src/utils/theme.go` to derive the theme from environment variables and default to "auto" if not set.
* Update `src/routes/home.go` to import and use `GetThemeFromEnv` in the `GetHome` function.
* Update `src/routes/options.go` to import and use `GetThemeFromEnv` in the `ChangeOptions` function.
* Update `src/routes/question.go` to import and use `GetThemeFromEnv` in the `ViewQuestion` function.


---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/httpjamesm/AnonymousOverflow/pull/145?shareId=a0dab6f3-027c-4f6e-85fe-60e7675d0e70).

src/routes/home.go

@@ -2,6 +2,7 @@ package routes
 
 import (
 	"anonymousoverflow/config"
+	"anonymousoverflow/src/utils"
 	"fmt"
 	"os"
 	"regexp"
@@ -11,10 +12,7 @@ import (
 )
 
 func GetHome(c *gin.Context) {
-	theme := os.Getenv("THEME")
+	theme := utils.GetThemeFromEnv()
-	if theme == "" {
-		theme = "auto"
-	}
 	c.HTML(200, "home.html", gin.H{
 		"version": config.Version,
 		"theme":   theme,

src/routes/options.go

@@ -2,10 +2,8 @@ package routes
 
 import (
 	"anonymousoverflow/config"
+	"anonymousoverflow/src/utils"
 	"fmt"
-	"os"
-
-	"github.com/gin-gonic/gin"
 )
 
 func ChangeOptions(c *gin.Context) {
@@ -18,10 +16,7 @@ func ChangeOptions(c *gin.Context) {
 			text = "enabled"
 		}
 		c.SetCookie("disable_images", fmt.Sprintf("%t", !c.MustGet("disable_images").(bool)), 60*60*24*365*10, "/", "", false, true)
-		theme := os.Getenv("THEME")
+		theme := utils.GetThemeFromEnv()
-		if theme == "" {
-			theme = "auto"
-		}
 		c.HTML(200, "home.html", gin.H{
 			"successMessage": "Images are now " + text,
 			"version":        config.Version,

src/routes/question.go

@@ -101,10 +101,7 @@ func ViewQuestion(c *gin.Context) {
 		imagePolicy = "'self'"
 	}
 
-	theme := os.Getenv("THEME")
+	theme := utils.GetThemeFromEnv()
-	if theme == "" {
-		theme = "auto"
-	}
 
 	c.HTML(200, "question.html", gin.H{
 		"question":    newFilteredQuestion,

src/utils/theme.go

@@ -0,0 +1,11 @@
+package utils
+
+import "os"
+
+func GetThemeFromEnv() string {
+	theme := os.Getenv("THEME")
+	if theme == "" {
+		theme = "auto"
+	}
+	return theme
+}