Move files for better organization

src/Helpers/ChangeNotifier.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using System.Text;
+
+namespace ReceiptPDFBuilder.Helpers
+{
+
+    // https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.inotifypropertychanged?view=netframework-4.7.2
+    class ChangeNotifier : INotifyPropertyChanged
+    {
+        public event PropertyChangedEventHandler? PropertyChanged;
+        protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
+        {
+            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+        }
+    }
+}

src/Helpers/Constants.cs

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+
+using System;
+
+namespace ReceiptPDFBuilder.Helpers;
+
+class Constants
+{
+    public static string AppVersion = "1.1.0";
+
+    public static string[] GetQuotes()
+    {
+        // sources:
+        // https://www.reddit.com/r/dadjokes/comments/1ehid23/dads_of_reddit_whats_a_short_clean_joke_that/
+        // https://www.thepioneerwoman.com/home-lifestyle/a35617884/best-dad-jokes/
+        // https://www.today.com/life/dad-jokes-rcna27325
+        // https://www.microscooters.com.au/blogs/family/100-dad-jokes-that-are-the-best-worst-in-the-book?srsltid=AfmBOopcTDq26iDYUsqaTjvUcVW6yxE-u942tatHC7Arns85unMMNfEO
+        return [
+            "When in the crucible of life, always remember to take your friends with you.",
+            "What do you call a paper airplane that won't fly? Stationary.",
+            "I used to be addicted to dad jokes, but now I'm all groan up.",
+            "I used to have a phobia about speed bumps. But I'm slowly getting over it.",
+            "Be careful trusting stairs. They're always up to something.",
+            "What do you call a fish with no eye? A fsh",
+            "How do you throw a party in outer space? You planet.",
+            "Why don't eggs tell jokes? They might crack up!",
+            "What do you call a snowman with a six-pack? An abdominal snowman!",
+            "Why did the math book look sad? Because it had too many problems!",
+            "I was going to tell you a joke about time travel, but you didn't like it.",
+            "I'm writing a book about glue, but I'm stuck on the first chapter.",
+            "If two vegetarians get in an argument, is it still called beef?",
+            "How do you stop a bull from charging? Cancel its credit card.",
+            "Why do seagulls fly over the sea? If they flew over the bay, they would be bagels.",
+            "What vegetable is cool, but not *that* cool? Radish.",
+            "How you fix a broken pumpkin? With a pumpkin patch.",
+            "Where do boats go when they're sick? To the dock.",
+            "Why was the broom late to class? It over-swept.",
+            "Wanna hear a joke about construction? I'm still workin' on it!",
+            "Which state has the most streets? Rhode Island.",
+            "What kind of car does a sheep like to drive? A lamborghini.",
+            "Where do surfers go for an education? Boarding school.",
+            "What do you get when you cross a fish with an elephant? Swimming trunks.",
+            "What did one eye say to the other? “Between us, something smells.”",
+            "When does a joke become a dad joke? When the punchline becomes apparent.",
+            "What do you call a cold puppy? A chili dog.",
+            "Why did the spider go to school? He wanted to be a web designer.",
+            "I was going to tell a sodium joke, then I thought, “Na.”",
+            "Did you hear about the two rowboats that got into an argument? It was an oar-deal.",
+            "What do you call birds that stick together? Velcrows.",
+            "How do birds learn to fly? They wing it.",
+            "Humpty Dumpty had a great fall. Summer wasn't too bad either.",
+            "How does the moon cut his hair? Eclipse it!",
+            "Why did the candle quit his job? He felt burned out."
+        ];
+    }
+}

src/Helpers/DataGridDropHandler.cs

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+using System.Collections.ObjectModel;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.VisualTree;
+using Avalonia.Xaml.Interactions.DragAndDrop;
+using ReceiptPDFBuilder.Models;
+using ReceiptPDFBuilder.ViewModels;
+
+namespace ReceiptPDFBuilder.Helpers;
+
+class DataGridDropHandler : BaseDataGridDropHandler<ReportFile>
+{
+    // https://wieslawsoltes.github.io/Xaml.Behaviors/articles/drag-and-drop-datagrid/datagrid-drag-and-drop-overview.html
+    // ...that page's code basically doesn't work in the way you'd expect. so just use the source code sample's code.
+    protected override bool Validate(DataGrid dg, DragEventArgs e, object? sourceContext, object? targetContext, bool execute)
+    {
+        if (sourceContext is not ReportFile sourceItem
+            || targetContext is not MainViewModel vm
+            || dg.GetVisualAt(e.GetPosition(dg)) is not Control targetControl
+            || targetControl.DataContext is not ReportFile targetItem)
+        {
+            return false;
+        }
+
+        var items = dg.ItemsSource as ObservableCollection<ReportFile>;
+        return RunDropAction(dg, e, execute, sourceItem, targetItem, items?? []);
+    }
+
+    protected override ReportFile MakeCopy(ObservableCollection<ReportFile> parentCollection, ReportFile item)
+    {
+        // Return a clone of the item if you support Copy operations
+        return new ReportFile { };
+    }
+}

src/Helpers/ListExtensions.cs

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+using System;
+using System.Collections.Generic;
+using System.Threading;
+
+namespace ReceiptPDFBuilders.Helpers;
+
+public static class ThreadSafeRandom
+{
+    [ThreadStatic] private static Random? Local;
+
+    public static Random ThisThreadsRandom
+    {
+        get { return Local ?? (Local = new Random(unchecked(Environment.TickCount * 31 + Thread.CurrentThread.ManagedThreadId))); }
+    }
+}
+
+// https://stackoverflow.com/a/1262619/3938401
+static class ListExtensions
+{
+    public static void Shuffle<T>(this IList<T> list)
+    {
+        int n = list.Count;
+        while (n > 1)
+        {
+            n--;
+            int k = ThreadSafeRandom.ThisThreadsRandom.Next(n + 1);
+            T value = list[k];
+            list[k] = list[n];
+            list[n] = value;
+        }
+    }
+}

src/Helpers/SourceGenerationContext.cs

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+using System.Text.Json.Serialization;
+using ReceiptPDFBuilder.Models;
+
+namespace ReceiptPDFBuilder.Helpers;
+
+[JsonSerializable(typeof(Settings))]
+[JsonSerializable(typeof(ReportFile))]
+[JsonSerializable(typeof(PDFReport))]
+internal partial class SourceGenerationContext : JsonSerializerContext { }

src/Helpers/Utilities.cs

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+
+using System;
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Text.RegularExpressions;
+
+namespace ReceiptPDFBuilders.Helpers;
+
+class Utilities
+{
+    public static JsonSerializerOptions GetSerializerOptions()
+    {
+        var opts = new JsonSerializerOptions 
+        { 
+            WriteIndented = false,
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        };
+        return opts;
+    }
+
+    public static DateOnly? CheckValidDateInString(string str)
+    {
+        // https://stackoverflow.com/a/14918404/3938401
+        var rgx = new Regex(@"\d{4}-\d{2}-\d{2}");
+        var mat = rgx.Match(str);
+        if (mat.Success)
+        {
+            var dtStr = mat.ToString();
+            string[] formats = ["yyyy-MM-dd"];
+            DateTime parsedDateTime;
+            var didWork = DateTime.TryParseExact(dtStr, formats, CultureInfo.InvariantCulture,
+                                        DateTimeStyles.None, out parsedDateTime);
+            return didWork ? DateOnly.FromDateTime(parsedDateTime) : null;
+        }
+        return null;
+    }
+}