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

DeepDrftPublic.Client/Common/PublicSiteSettings.cs

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+using Microsoft.AspNetCore.Components;
+
+namespace DeepDrftPublic.Client.Common;
+
+/// <summary>
+/// The single public-site listener-settings object (Phase 18 wave 18.6, §4a). The generalized analogue of
+/// <see cref="DarkModeSettings"/>: one scoped holder for every remembered listener preference, seeded at
+/// server prerender, carried into WASM via <see cref="PersistentState"/>, and persisted to a cookie on
+/// change. Today it carries one preference — streaming quality; tomorrow dark mode (and whatever follows)
+/// folds in here as another property without disturbing the menu that reads it.
+/// <para>
+/// Built design-for-adaptability per §4a: a new preference is a new <c>[PersistentState]</c> property here
+/// plus a new <see cref="Components.SettingsItem"/> in the menu — not a rewire. Dark mode is intentionally
+/// <em>not</em> migrated in now (it keeps its own <see cref="DarkModeSettings"/> seam); this object is shaped
+/// so that consolidation is later a merge of two identical seams, not a reconciliation of two different ones.
+/// </para>
+/// </summary>
+public class PublicSiteSettings
+{
+    /// <summary>
+    /// The listener's streaming-quality preference. Defaults to <see cref="StreamQuality.LowData"/> (Opus,
+    /// capability-gated — OQ2). Seeded from the <c>streamQuality</c> cookie at prerender; persisted on change
+    /// by the client cookie service. The player reads this to decide which <c>?format=</c> to request, but
+    /// the capability gate and C2 fallback still apply on top, so a <see cref="StreamQuality.LowData"/>
+    /// preference never forces an unplayable stream.
+    /// </summary>
+    [PersistentState]
+    public StreamQuality StreamQuality { get; set; } = StreamQuality.LowData;
+}

DeepDrftPublic.Client/Common/SettingsItem.cs

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+using Microsoft.AspNetCore.Components;
+
+namespace DeepDrftPublic.Client.Common;
+
+/// <summary>
+/// One entry in the public-site Settings menu (Phase 18 wave 18.6, §4a). The settings-item abstraction the
+/// menu renders instead of a hard-coded control list: a <see cref="Label"/> plus a <see cref="Control"/>
+/// fragment bound to a persisted preference. Adding a future tenant (e.g. dark mode) is appending one of
+/// these — not rewiring the menu. The control fragment owns its own binding to <see cref="PublicSiteSettings"/>
+/// and its own persistence call, so each item is self-contained and the menu stays preference-agnostic.
+/// </summary>
+public sealed record SettingsItem(string Label, RenderFragment Control);

DeepDrftPublic.Client/Common/StreamQuality.cs

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+namespace DeepDrftPublic.Client.Common;
+
+/// <summary>
+/// The listener's streaming-quality preference (Phase 18 wave 18.6, §4). This is the user's <em>intent</em>,
+/// not the wire format that ultimately gets served: <see cref="LowData"/> means "give me Opus if you can,"
+/// but the player still capability-gates and C2-falls-back to lossless when Opus can't play (a browser that
+/// can't decode Ogg Opus, or a track with no Opus artifact). It is therefore deliberately distinct from
+/// <c>DeepDrftModels.Enums.AudioFormat</c> (the delivery rendering resolved per request): one is the
+/// remembered preference, the other is what a given stream request actually asks for.
+/// </summary>
+public enum StreamQuality
+{
+    /// <summary>Bandwidth-friendly Opus (capability-gated; the default before any choice — OQ2).</summary>
+    LowData,
+
+    /// <summary>The lossless WAV path, always playable everywhere.</summary>
+    Lossless
+}