Add news feature with CRUD operations and real-time notifications

- Implemented news management functionality, allowing admins to create, read, update, and delete news posts.
- Introduced a new SignalR event for broadcasting news updates to all connected clients.
- Updated API documentation to include new endpoints for news management.
- Enhanced frontend with a dedicated news page and admin interface for managing news posts.
- Added necessary localization for news-related terms in both Russian and English.

backend/src/PnvPanel.Application/News/ListNewsQuery.cs

@@ -0,0 +1,6 @@
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.News;
+
+public sealed record ListNewsQuery(int Page, int PageSize) : IQuery<Result<PagedList<NewsPostDto>>>;

backend/src/PnvPanel.Application/News/ListNewsQueryHandler.cs

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+using Microsoft.EntityFrameworkCore;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.News;
+
+public sealed class ListNewsQueryHandler(IAppDbContext dbContext) : IQueryHandler<ListNewsQuery, Result<PagedList<NewsPostDto>>>
+{
+    public async Task<Result<PagedList<NewsPostDto>>> Handle(ListNewsQuery query, CancellationToken cancellationToken)
+    {
+        var page = query.Page <= 0 ? 1 : query.Page;
+        var pageSize = query.PageSize is <= 0 or > 100 ? 20 : query.PageSize;
+
+        var result = await dbContext.NewsPosts.AsNoTracking()
+            .OrderByDescending(p => p.CreatedAt)
+            .Select(p => new NewsPostDto(p.Id, p.Title, p.Body, p.CreatedAt, p.UpdatedAt))
+            .ToPagedListAsync(page, pageSize, cancellationToken);
+
+        return Result.Success(result);
+    }
+}

backend/src/PnvPanel.Application/News/NewsPostDto.cs

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+using PnvPanel.Domain.News;
+
+namespace PnvPanel.Application.News;
+
+public sealed record NewsPostDto(Guid Id, string Title, string Body, DateTimeOffset CreatedAt, DateTimeOffset? UpdatedAt)
+{
+    public static NewsPostDto FromDomain(NewsPost post) => new(post.Id, post.Title, post.Body, post.CreatedAt, post.UpdatedAt);
+}