feat: add application versioning support to backend and UI

- Introduce APP_VERSION build argument and environment variable in Dockerfile.
- Load AppVersion from environment variables in the configuration loader.
- Pass the application version to the HTML renderer and expose it to templates via PageData.
- Update tests to verify the version is correctly rendered in the footer.

backend/libs/handlers/accounts_test.go

@@ -569,6 +569,9 @@ func TestHomeReflectsUploadPolicySettings(t *testing.T) {
 	if !strings.Contains(body, "Max file size: 123 MB") || !strings.Contains(body, "456 MB") {
 		t.Fatalf("home did not reflect policy settings: %s", body)
 	}
+	if !strings.Contains(body, "warpbox.dev · test ·") {
+		t.Fatalf("home footer did not include app version: %s", body)
+	}
 }
 
 func TestAPIDocsHeaderReflectsLoggedInUser(t *testing.T) {