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/Pages/MixesView.razor

@@ -0,0 +1,15 @@
+@page "/mixes"
+@using DeepDrftPublic.Client.Controls
+@inherits MediumBrowseBase
+
+<PageTitle>DeepDrft Mixes</PageTitle>
+
+<ReleaseGallery Releases="@Releases"
+                Loading="@Loading"
+                DetailRoute="mixes"
+                EmptyMessage="No mixes yet" />
+
+@code {
+    protected override DeepDrftModels.Enums.ReleaseMedium Medium => DeepDrftModels.Enums.ReleaseMedium.Mix;
+    protected string DetailRoute => "mixes";
+}