remediate: replace eval cookie writes with safe JS helper + add tests (18.6 Track A)

Both SettingsCookieService and DarkModeCookieService now call window.DeepDrftSettings.setCookie (new Interop/settings/settings.ts) instead of eval. New tests cover SettingsServiceBase parse/format round-trip and the PreferenceAwareStreamingPlayerService invariant (Lossless skips probe; LowData inherits base).

DeepDrftPublic.Client/Services/DarkModeCookieService.cs

@@ -14,9 +14,7 @@ public class DarkModeCookieService(DarkModeSettings darkModeSetting, IJSRuntime
 
     public async ValueTask SetDarkModeAsync(bool isDarkMode)
     {
-        var expires = DateTime.UtcNow.AddDays(EXPIRY_DAYS).ToString("R");
-        await js.InvokeVoidAsync("eval", 
-            $"document.cookie = '{COOKIE_NAME}={isDarkMode.ToString().ToLower()}; expires={expires}; path=/; SameSite=Lax'");
+        await js.InvokeVoidAsync("DeepDrftSettings.setCookie", COOKIE_NAME, isDarkMode.ToString().ToLower(), EXPIRY_DAYS);
         darkModeSetting.IsDarkMode = isDarkMode;
     }
 }

DeepDrftPublic.Client/Services/SettingsCookieService.cs

@@ -27,8 +27,6 @@ public class SettingsCookieService(PublicSiteSettings settings, IJSRuntime js) :
 
     private async ValueTask WriteCookieAsync(string name, string value)
     {
-        var expires = DateTime.UtcNow.AddDays(ExpiryDays).ToString("R");
-        await js.InvokeVoidAsync("eval",
-            $"document.cookie = '{name}={value}; expires={expires}; path=/; SameSite=Lax'");
+        await js.InvokeVoidAsync("DeepDrftSettings.setCookie", name, value, ExpiryDays);
     }
 }