feat(centralui): collapsible sidebar nav sections

Make the seven sidebar section groups (Admin, Design, Deployment,
Notifications, Site Calls, Monitoring, Audit) collapsible. New NavSection
component renders a header toggle button (chevron) and reveals its items
only while expanded; NavMenu owns the expanded-section set.

Behaviour: sections are collapsed by default; state persists in the
`scadabridge_nav` cookie (written/read via the new nav-state.js JS interop,
mirroring treeview-storage.js) so it survives reloads and reconnects;
navigating into a section auto-expands it and remembers it. The Dashboard
item stays sectionless and always visible.

Tests: NavMenu bUnit tests expand sections before asserting items and add
collapsed-by-default / toggle / cookie-persistence cases; Playwright nav
tests expand sections before clicking links; new NavCollapseTests covers
the feature E2E. Build 0 warnings; bUnit 545 passed; Playwright nav suite
green (the unrelated AuditGridColumnTests resize-reload case remains
pre-existing flaky — an un-awaited save race in that test).

src/ScadaLink.CentralUI/wwwroot/css/site.css

@@ -42,7 +42,17 @@
     padding-left: calc(1rem - 3px);
 }
 
-.sidebar .nav-section-header {
+/* Collapsible section header — a full-width button styled as an uppercase
+   eyebrow with a leading expand/collapse chevron. */
+.sidebar .nav-section-toggle {
+    display: flex;
+    align-items: center;
+    gap: 0.4rem;
+    width: 100%;
+    background: none;
+    border: 0;
+    cursor: pointer;
+    text-align: left;
     color: var(--ink-faint);
     font-size: 0.7rem;
     font-weight: 600;
@@ -52,6 +62,15 @@
     margin-top: 0.5rem;
 }
 
+.sidebar .nav-section-toggle:hover {
+    color: var(--ink);
+}
+
+.sidebar .nav-section-toggle .bi {
+    font-size: 0.8rem;
+    line-height: 1;
+}
+
 .sidebar .brand {
     color: var(--ink);
     font-size: 1.1rem;