Phase 9 Wave 4: ARCHIVE nav + Cuts/Sessions/Mixes pages + MixWaveformVisualizer

Replaces flat RELEASES/SESSIONS/MIXES nav with ARCHIVE dropdown (PageRoute.Children,
one-level cap, dual-role node). Adds /archive overview, /cuts (AlbumsView + medium
filter; /albums redirects), /sessions + /sessions/{id} (hero-dominant), /mixes +
/mixes/{id} (MixWaveformVisualizer full-page background). Extracts ReleaseDetailScaffold
from TrackDetail (invariant trio). PersistentComponentState bridge on all new pages.
Click-to-seek seam designed on MixWaveformVisualizer (inert until wired).

DeepDrftPublic.Client/Layout/DeepDrftMenu.razor

@@ -10,21 +10,33 @@
         <ul class="dd-nav-links">
             @foreach (var navPage in Pages.MenuPages)
             {
-                <li>
-                    <a href="@navPage.Route" class="dd-nav-link">@navPage.Name</a>
-                </li>
+                @if (navPage.HasChildren)
+                {
+                    @* Dual-role node: the parent anchor navigates to its own route on click,
+                       while hover/focus reveals the child dropdown (pure CSS, no JS). *@
+                    <li class="dd-nav-item-parent">
+                        <a href="@navPage.Route" class="dd-nav-link">@navPage.Name</a>
+                        <ul class="dd-nav-dropdown">
+                            @foreach (var child in navPage.Children)
+                            {
+                                <li>
+                                    <a href="@child.Route" class="dd-nav-link dd-nav-dropdown-link">@child.Name</a>
+                                </li>
+                            }
+                        </ul>
+                    </li>
+                }
+                else
+                {
+                    <li>
+                        <a href="@navPage.Route" class="dd-nav-link">@navPage.Name</a>
+                    </li>
+                }
             }
         </ul>
 
         <div class="dd-nav-actions">
             <StreamNowButton ButtonClass="dd-nav-cta" ButtonLabel="Stream Now &#9654;" />
-            @* <button type="button" *@
-            @*         class="dd-nav-toggle" *@
-            @*         aria-label="Toggle dark mode" *@
-            @*         aria-pressed="@IsDarkMode.ToString().ToLowerInvariant()" *@
-            @*         @onclick="DarkModeToggle"> *@
-            @*     @((MarkupString)DarkLightModeIconSvg) *@
-            @* </button> *@
         </div>
     </nav>
 </div>
@@ -35,13 +47,6 @@
         <a class="dd-nav-brand" href="/">Deep DRFT</a>
         
         <div class="dd-nav-actions">
-            @* <button type="button" *@
-            @*         class="dd-nav-toggle" *@
-            @*         aria-label="Toggle dark mode" *@
-            @*         aria-pressed="@IsDarkMode.ToString().ToLowerInvariant()" *@
-            @*         @onclick="DarkModeToggle"> *@
-            @*     @((MarkupString)DarkLightModeIconSvg) *@
-            @* </button> *@
             <button type="button"
                     class="dd-nav-hamburger"
                     aria-label="Toggle navigation"
@@ -59,6 +64,13 @@
                     <li>
                         <a href="@navPage.Route" class="dd-nav-link" @onclick="CloseMobileMenu">@navPage.Name</a>
                     </li>
+                    @* One-level fan-out: render children as an indented sub-list under the parent link. *@
+                    @foreach (var child in navPage.Children)
+                    {
+                        <li class="dd-nav-mobile-child">
+                            <a href="@child.Route" class="dd-nav-link" @onclick="CloseMobileMenu">@child.Name</a>
+                        </li>
+                    }
                 }
                 <li>
                     <StreamNowButton ButtonClass="dd-nav-cta" ButtonLabel="Stream Now &#9654;" OnStreamStarted="CloseMobileMenu" />