Add data models, randomization, and report factory setup

Introduced several new classes in the `EnvelopeGenerator.WebUI.Client` namespace:
- Added `Adjustment` class for financial adjustments with deterministic randomization.
- Added `Customer` class to load customer data from a SQL data source with fallback.
- Added `DataItem` class to represent detailed billing data, including adjustments.
- Added `DataItemList` class implementing `IList` for dynamic `DataItem` generation.
- Added `DeterministicRandom` class for reproducible random value generation.
- Added `Term` struct to define payment terms.
- Added `ReportsFactory` class to manage predefined reports.

Updated `MIGRATION_CONTEXT.md` to document the completion of Phase 5 (Data & PredefinedReports Migration) and outline next steps for resolving DevExpress-related errors in Phase 7.

EnvelopeGenerator.WebUI/EnvelopeGenerator.WebUI.Client/Data/Term.cs

@@ -0,0 +1,15 @@
+namespace EnvelopeGenerator.WebUI.Client.Data {
+    public struct Term {
+        public static readonly Term[] Terms = new Term[] {
+            new Term("Payment seven days after invoice date" ),
+            new Term("Payment ten days after invoice date" ),
+            new Term("End of month" ),
+            new Term("21st of the month following invoice date" ),
+        };
+        readonly string currentName;
+        public string Name { get { return currentName; } }
+        public Term(string currentName) {
+            this.currentName = currentName;
+        }
+    }
+}