Add instructions management functionality and update related components

- Introduced new endpoints for managing instruction intros and tabs, allowing admins to create, update, and delete instructional content.
- Enhanced the FactoryResetCommandHandler to include the seeding of instruction data during a factory reset.
- Updated the database schema to include InstructionIntro and InstructionTab entities, with corresponding migrations.
- Improved frontend routing and components to support the new instructions section, including a dedicated page for displaying instructions and tabs.
- Enhanced API documentation to reflect the new instruction management features and their expected request/response formats.
- Added localization support for the new instructions functionality in both Russian and English.

backend/tests/PnvPanel.Application.Tests/Instructions/GetInstructionIntroQueryHandlerTests.cs

@@ -0,0 +1,37 @@
+using PnvPanel.Application.Instructions;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Instructions;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Instructions;
+
+public class GetInstructionIntroQueryHandlerTests
+{
+    [Fact]
+    public async Task Handle_WhenNoIntroSeeded_ReturnsEmptyDefault()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var handler = new GetInstructionIntroQueryHandler(dbContext);
+
+        var result = await handler.Handle(new GetInstructionIntroQuery(), CancellationToken.None);
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(string.Empty, result.Value.Body);
+    }
+
+    [Fact]
+    public async Task Handle_WhenIntroExists_ReturnsIt()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var intro = InstructionIntro.Create("Привет, это инструкция.");
+        dbContext.InstructionIntros.Add(intro);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var handler = new GetInstructionIntroQueryHandler(dbContext);
+
+        var result = await handler.Handle(new GetInstructionIntroQuery(), CancellationToken.None);
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal("Привет, это инструкция.", result.Value.Body);
+    }
+}