feat(sender): add create-envelope wizard with bootstrap handoff

Introduce a 4-step modal wizard on SenderPage for creating envelopes.

Validate required fields, support optional skip, receiver management, PDF upload, and keyboard shortcuts.

Persist wizard payload in sessionStorage via token-based bootstrap service and navigate to editor with bootstrapToken.

Register bootstrap storage service in both client and server DI containers and add dedicated sender wizard styling.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/CreateEnvelopeBootstrapStorageService.cs

@@ -0,0 +1,54 @@
+using System.Text.Json;
+using Microsoft.JSInterop;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+public sealed class CreateEnvelopeBootstrapStorageService(IJSRuntime jsRuntime)
+{
+    private const string StorageKeyPrefix = "sender.createEnvelope.bootstrap.";
+    private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web);
+
+    public async Task<string> StoreAsync(CreateEnvelopeBootstrapPayload payload)
+    {
+        var token = Guid.NewGuid().ToString("N");
+        var key = BuildStorageKey(token);
+        var json = JsonSerializer.Serialize(payload, JsonOptions);
+        await jsRuntime.InvokeVoidAsync("sessionStorage.setItem", key, json);
+        return token;
+    }
+
+    public async Task<CreateEnvelopeBootstrapPayload?> ConsumeAsync(string? token)
+    {
+        if (string.IsNullOrWhiteSpace(token))
+            return null;
+
+        var key = BuildStorageKey(token);
+        var raw = await jsRuntime.InvokeAsync<string?>("sessionStorage.getItem", key);
+        await jsRuntime.InvokeVoidAsync("sessionStorage.removeItem", key);
+
+        if (string.IsNullOrWhiteSpace(raw))
+            return null;
+
+        try
+        {
+            return JsonSerializer.Deserialize<CreateEnvelopeBootstrapPayload>(raw, JsonOptions);
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    private static string BuildStorageKey(string token) => $"{StorageKeyPrefix}{token}";
+}
+
+public sealed record CreateEnvelopeBootstrapPayload(
+    string Title,
+    string Message,
+    int? EnvelopeTypeId,
+    string Language,
+    IReadOnlyList<CreateEnvelopeBootstrapReceiver> Receivers,
+    byte[]? DocumentBytes,
+    string? DocumentFileName);
+
+public sealed record CreateEnvelopeBootstrapReceiver(string FullName, string EmailAddress);