feat(front-hall): implement three-mode layout with quick links panel

Phase 2 of Organizr Migration - Front Hall restructure:

- Add FrontHallState provider with three modes (dashboard, iframe, settings)
- Create QuickLinksPanel widget with categorized links and overflow menus
- Add IframeView with platform-aware implementation (web iframe, mobile fallback)
- Extract DashboardContent from FrontHallPage
- Add QuickLinkSettingsContent placeholder for Phase 4 link editor
- Implement QuickLink entity and data layer with Core API datasource
- Add default quick links fallback when API unavailable
- Update UI_LAYOUT.md with Front Hall panel configurations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/features/front_hall/domain/entities/quick_link.dart

@@ -0,0 +1,53 @@
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+part 'quick_link.freezed.dart';
+
+/// Quick link type for navigation behavior.
+enum QuickLinkType {
+  /// Opens in embedded iframe within the app.
+  iframe,
+
+  /// Opens in a new browser tab.
+  newTab,
+}
+
+/// Quick link entity for Front Hall navigation.
+///
+/// Represents a shortcut link that can either open embedded content
+/// in an iframe or navigate to an external URL in a new tab.
+@freezed
+sealed class QuickLink with _$QuickLink {
+  const factory QuickLink({
+    /// Unique identifier.
+    required String id,
+
+    /// Display name.
+    required String name,
+
+    /// Target URL.
+    required String url,
+
+    /// Material icon name (e.g., 'home', 'settings', 'movie').
+    required String iconName,
+
+    /// Category for grouping (e.g., 'Home', 'AMP', 'Coding', 'Infrastructure').
+    String? category,
+
+    /// Navigation behavior type.
+    @Default(QuickLinkType.iframe) QuickLinkType type,
+
+    /// Sort order within category.
+    @Default(0) int sortOrder,
+
+    /// Whether the link is active/visible.
+    @Default(true) bool isActive,
+  }) = _QuickLink;
+
+  const QuickLink._();
+
+  /// Whether this link opens in an iframe.
+  bool get isIframe => type == QuickLinkType.iframe;
+
+  /// Whether this link opens in a new tab.
+  bool get isNewTab => type == QuickLinkType.newTab;
+}