feat(notifications): day sheet notification types

Add daySheetId field to NotificationItem model (Brick-managed), extract
notificationTitle/notificationIcon to top-level testable functions, add
five day-sheet notification cases, and wire tap/banner routing for
day-sheet detail and approvals tab.
Co-Authored-By: claude-flow <ruv@ruv.net>

test/notification_titles_test.dart

@@ -0,0 +1,87 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tasq/screens/notifications/notifications_screen.dart';
+
+void main() {
+  group('notificationTitle', () {
+    test('assignment guard (existing case unchanged)', () {
+      expect(notificationTitle('assignment', 'Alice'), 'Alice assigned you');
+    });
+
+    test('day_sheet_submitted', () {
+      expect(
+        notificationTitle('day_sheet_submitted', 'Bob'),
+        'Bob submitted a day sheet',
+      );
+    });
+
+    test('day_sheet_disapproved', () {
+      expect(
+        notificationTitle('day_sheet_disapproved', 'Alice'),
+        'Day sheet disapproved',
+      );
+    });
+
+    test('day_sheet_justified', () {
+      expect(
+        notificationTitle('day_sheet_justified', 'Bob'),
+        'Bob justified a day sheet',
+      );
+    });
+
+    test('day_sheet_approved', () {
+      expect(
+        notificationTitle('day_sheet_approved', 'Alice'),
+        'Day sheet approved',
+      );
+    });
+
+    test('day_sheet_digest', () {
+      expect(
+        notificationTitle('day_sheet_digest', 'System'),
+        'Day sheets awaiting review',
+      );
+    });
+  });
+
+  group('notificationIcon', () {
+    test('assignment guard (existing case unchanged)', () {
+      expect(notificationIcon('assignment'), Icons.assignment_ind_outlined);
+    });
+
+    test('day_sheet_submitted', () {
+      expect(
+        notificationIcon('day_sheet_submitted'),
+        Icons.assignment_turned_in_outlined,
+      );
+    });
+
+    test('day_sheet_disapproved', () {
+      expect(
+        notificationIcon('day_sheet_disapproved'),
+        Icons.report_outlined,
+      );
+    });
+
+    test('day_sheet_justified', () {
+      expect(
+        notificationIcon('day_sheet_justified'),
+        Icons.reply_outlined,
+      );
+    });
+
+    test('day_sheet_approved', () {
+      expect(
+        notificationIcon('day_sheet_approved'),
+        Icons.verified_outlined,
+      );
+    });
+
+    test('day_sheet_digest', () {
+      expect(
+        notificationIcon('day_sheet_digest'),
+        Icons.inbox_outlined,
+      );
+    });
+  });
+}