feat(adminui): collapsible nav sidebar with cookie state + LoginLayout

Port the ScadaLink CentralUI sidebar pattern into the OtOpcUa AdminUI:

- Drop the top app-bar. Brand moves into the side rail's header — same
  visual rhythm as ScadaLink's NavMenu.
- New NavSection.razor: collapsible eyebrow toggle (rail-eyebrow-toggle CSS)
  with a chevron + label. Mirrors ScadaLink/Components/Layout/NavSection.
- New NavSidebar.razor: interactive island carrying the three section
  groups (Navigation / Scripting / Live) + session block. Marked
  @rendermode InteractiveServer; MainLayout itself stays static-rendered
  because layouts can't take a RenderFragment Body across an interactive
  boundary.
- New wwwroot/js/nav-state.js: window.navState.get/.set persists the
  expanded-section list to the otopcua_nav cookie (one-year lifetime,
  SameSite=Lax). Same shape as ScadaLink's scadabridge_nav.
- New LoginLayout.razor + @layout LoginLayout on Login.razor: the login
  page now renders without the side rail — clean centred card.
- MainLayout.razor: slimmed down to the d-flex shell + hamburger toggle +
  <NavSidebar/> + @Body.
- Login.razor: also drops the trailing "LDAP bind against the configured
  directory..." footer that the user asked to remove.
- site.css: adds .side-rail .brand styles (mirrored from ScadaLink) and
  the .rail-eyebrow-toggle / .rail-eyebrow-chevron / .rail-section-body
  styles for the new collapsible UI.

Auto-expand on page load: NavSidebar seeds the expanded set from the
current URL's first path segment (in OnInitialized so it works even on
the very first server render) and from the cookie (in OnAfterRenderAsync
once JS interop is available). LocationChanged hooks keep the expanded
state in sync as the user navigates between sections.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/wwwroot/css/site.css

@@ -49,6 +49,19 @@
     }
 }
 
+/* Brand block pinned at the top of the side rail. Mirrors ScadaLink's
+   .sidebar .brand styling — used now that the top app-bar was dropped. */
+.side-rail .brand {
+    color: var(--ink);
+    font-size: 1.1rem;
+    font-weight: 600;
+    letter-spacing: 0.02em;
+    padding: 1rem;
+    border-bottom: 1px solid var(--rule);
+    margin-bottom: 0.4rem;
+}
+.side-rail .brand .mark { color: var(--accent); }
+
 .rail-eyebrow {
     font-size: 0.68rem;
     font-weight: 600;
@@ -58,6 +71,36 @@
     padding: 0.3rem 0.6rem;
 }
 
+/* Collapsible variant — rendered by NavSection.razor. Looks like .rail-eyebrow
+   plus a leading chevron; clicking flips chevron + expanded state. */
+.rail-eyebrow-toggle {
+    display: flex;
+    align-items: center;
+    gap: 0.4rem;
+    width: 100%;
+    background: transparent;
+    border: 0;
+    text-align: left;
+    font-size: 0.68rem;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.07em;
+    color: var(--ink-faint);
+    padding: 0.45rem 0.6rem 0.3rem;
+    cursor: pointer;
+}
+.rail-eyebrow-toggle:hover { color: var(--ink); }
+.rail-eyebrow-chevron {
+    display: inline-block;
+    width: 0.7rem;
+    font-size: 0.55rem;
+    color: var(--ink-faint);
+}
+.rail-section-body {
+    display: flex;
+    flex-direction: column;
+}
+
 .rail-link {
     display: block;
     padding: 0.4rem 0.6rem;