feature: Phase 18.6 Track A — public Settings menu + streaming-quality toggle

DeepDrftPublic.Client/Services/SettingsCookieService.cs

@@ -0,0 +1,34 @@
+using DeepDrftPublic.Client.Common;
+using Microsoft.JSInterop;
+
+namespace DeepDrftPublic.Client.Services;
+
+/// <summary>
+/// Client-side runtime writer for public-site settings (Phase 18 wave 18.6), the analogue of
+/// <see cref="DarkModeCookieService"/>. Reads the current preference off the in-memory
+/// <see cref="PublicSiteSettings"/> (already seeded at prerender and bridged into WASM), and writes a
+/// 365-day cookie via <c>document.cookie</c> interop when the listener changes it in the Settings menu —
+/// the same durable-truth seam dark mode uses, so the choice survives the session and seeds the next visit's
+/// prerender (no flash).
+/// </summary>
+public class SettingsCookieService(PublicSiteSettings settings, IJSRuntime js) : SettingsServiceBase
+{
+    private const int ExpiryDays = 365;
+
+    public StreamQuality GetStreamQuality() => settings.StreamQuality;
+
+    public async ValueTask SetStreamQualityAsync(StreamQuality quality)
+    {
+        if (settings.StreamQuality == quality) return;
+
+        await WriteCookieAsync(StreamQualityCookieName, FormatStreamQuality(quality));
+        settings.StreamQuality = quality;
+    }
+
+    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'");
+    }
+}