add link picker and validation to navigation editor

- replace freeform inputs with grouped dropdown (pages, custom pages,
  collections, external URL)
- add inline URL validation for external links
- add inline feedback component instead of flash messages
- add dismiss-on-interaction pattern (feedback clears on changes)
- add no-JS fallback via NavigationController
- add DirtyGuard hook to warn before navigating away with unsaved changes
- add no-JS fallbacks for settings forms (from address, signing secret)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/admin/components.css

@@ -4629,6 +4629,37 @@
   color: var(--admin-text-faintest);
 }
 
+.nav-editor-select {
+  flex: 1;
+  min-width: 0;
+  font-size: 0.875rem;
+  padding: 0.375rem 0.5rem;
+}
+
+.nav-editor-item {
+  flex-wrap: wrap;
+}
+
+.nav-editor-external {
+  flex-basis: 100%;
+  padding-top: 0.5rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+}
+
+.nav-editor-error {
+  display: flex;
+  align-items: center;
+  gap: 0.25rem;
+  font-size: 0.75rem;
+  color: var(--t-text-error);
+}
+
+.admin-input-error {
+  border-color: var(--t-border-error);
+}
+
 /* ── Page editor ── */
 
 .admin-editor-badges {