add data-driven navigation with admin editor

Replace hardcoded header, footer and mobile nav with settings-driven
loops. Nav items stored as JSON via Settings, loaded in ThemeHook with
sensible defaults. New admin navigation editor at /admin/navigation
for add/remove/reorder/save/reset. Mobile bottom nav also driven from
header nav items with icon mapping by slug.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/admin/components.css

@@ -2152,4 +2152,71 @@
   to { opacity: 1; }
 }
 
+/* Navigation editor */
+
+.nav-editor-item {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.5rem;
+  border: 1px solid var(--t-border-default);
+  border-radius: 0.375rem;
+  background: var(--t-surface-base);
+}
+
+.nav-editor-fields {
+  flex: 1;
+  display: flex;
+  gap: 0.5rem;
+  min-width: 0;
+}
+
+.nav-editor-input {
+  flex: 1;
+  min-width: 0;
+  font-size: 0.875rem;
+  padding: 0.375rem 0.5rem;
+}
+
+.nav-editor-actions {
+  display: flex;
+  gap: 0.125rem;
+  flex-shrink: 0;
+}
+
+.nav-editor-dropdown {
+  position: absolute;
+  top: 100%;
+  left: 0;
+  z-index: 10;
+  margin-top: 0.25rem;
+  min-width: 12rem;
+  background: var(--t-surface-base);
+  border: 1px solid var(--t-border-default);
+  border-radius: 0.375rem;
+  box-shadow: 0 4px 6px -1px rgb(0 0 0 / 0.1);
+  overflow: hidden;
+}
+
+.nav-editor-dropdown-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.5rem;
+  width: 100%;
+  padding: 0.5rem 0.75rem;
+  font-size: 0.875rem;
+  text-align: left;
+  cursor: pointer;
+  background: none;
+  border: none;
+  color: var(--t-text-primary);
+
+  @media (hover: hover) {
+    &:hover {
+      background: var(--t-surface-sunken);
+    }
+  }
+}
+
 } /* @layer admin */