feat: 新增文件库功能，支持局域网文件浏览与媒体播放

后端：
- 新增 ManagedLibraryRoot / ManagedFileRecord 数据模型及 SQLite 迁移
- 新增文件库服务、端点服务及定时扫描后台任务
- 新增 REST API: drives、directories、roots CRUD、files 分页搜索、文本预览
- 新增文件流端点支持视频/音频流式传输
- 数据库切换为 SQLite，Kestrel 绑定 0.0.0.0 支持局域网访问

前端：
- 管理端：磁盘浏览、目录选择、根目录添加/启用/删除/扫描
- 客户端：根目录选择、文件搜索/筛选/分页、音视频播放、文本预览
- 全新响应式 UI（桌面+移动端），CSS 变量设计系统
- HTTP 客户端支持 Vite 开发代理与生产同源自动切换
- 移除 HTTPS 强制重定向以提升移动端视频流兼容性

Avalonia-API/Extensions/FileStreamEndpointExtensions.cs

@@ -0,0 +1,47 @@
+using Avalonia_EFCore.Database;
+using Microsoft.EntityFrameworkCore;
+
+namespace Avalonia_API.Extensions
+{
+    public static class FileStreamEndpointExtensions
+    {
+        public static IEndpointRouteBuilder MapFileStreamEndpoints(this IEndpointRouteBuilder app)
+        {
+            app.MapMethods("/api/files/{id:int}/stream", ["GET", "HEAD"], async (int id, AppDataContext db, HttpContext httpContext) =>
+            {
+                // Browsers cancel in-flight range requests aggressively while seeking.
+                // Keep this small metadata lookup independent from RequestAborted so
+                // EF does not throw TaskCanceledException before the file is opened.
+                var file = await db.ManagedFileRecords
+                    .AsNoTracking()
+                    .Include(item => item.LibraryRoot)
+                    .FirstOrDefaultAsync(item =>
+                        item.Id == id
+                        && item.Exists
+                        && item.LibraryRoot != null
+                        && item.LibraryRoot.IsAvailable);
+
+                if (file is null || !System.IO.File.Exists(file.AbsolutePath))
+                {
+                    return Results.NotFound();
+                }
+
+                var stream = System.IO.File.Open(file.AbsolutePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+                httpContext.Response.Headers.ContentDisposition = $"inline; filename=\"{Uri.EscapeDataString(file.FileName)}\"";
+                httpContext.Response.Headers.AcceptRanges = "bytes";
+                httpContext.Response.Headers.CacheControl = "public, max-age=3600";
+
+                return Results.File(
+                    stream,
+                    contentType: file.ContentType,
+                    fileDownloadName: null,
+                    lastModified: file.LastWriteTimeUtc,
+                    enableRangeProcessing: true);
+            })
+            .WithName("StreamManagedFile")
+            .WithTags("FileLibrary");
+
+            return app;
+        }
+    }
+}

Avalonia-API/Extensions/UnifiedEndpointExtensions.cs

@@ -131,9 +131,11 @@ namespace Avalonia_API.Extensions
         {
             return async (HttpContext httpContext) =>
             {
-                var ctx = await BuildContextFromHttpContext(httpContext);
+                var ctx = httpContext.Items["UnifiedContext"] as ServiceEndpointContext
+                    ?? await BuildContextFromHttpContext(httpContext);
                 ctx.Items["ServiceProvider"] = serviceProvider;
                 ctx.Items["User"] = httpContext.User;
+                httpContext.Items["UnifiedContext"] = ctx;
 
                 var result = await unifiedHandler(ctx);
 
@@ -204,6 +206,7 @@ namespace Avalonia_API.Extensions
 
             httpContext.Items["UnifiedContext"] = ctx;
 
+            object? nextResult = null;
             await unifiedFilter.InvokeAsync(ctx, async (c) =>
             {
                 httpContext.Response.StatusCode = c.StatusCode;
@@ -211,7 +214,7 @@ namespace Avalonia_API.Extensions
                 {
                     httpContext.Response.Headers[kvp.Key] = kvp.Value;
                 }
-                await aspNext(aspContext);
+                nextResult = await aspNext(aspContext);
             });
 
             if (ctx.ResponseBody is not null)
@@ -219,7 +222,7 @@ namespace Avalonia_API.Extensions
                 return Results.Json(ctx.ResponseBody, statusCode: ctx.StatusCode);
             }
 
-            return null!;
+            return nextResult;
         }
     }
 }