style(ui): 收窄宽屏留白——作品库加宽到 max-w-7xl + xl 四列，写作正文 720→768px

- PageContainer wide 档 max-w-6xl→max-w-7xl；作品库改用 wide 档
- 作品库卡片网格 lg 三列基础上加 xl:grid-cols-4，宽屏铺满少留白
- 写作台正文测量 720→768px（约 42 中文字/行，仍舒适），收窄两侧空档

apps/web/components/projects/ProjectLibrary.tsx

@@ -150,7 +150,7 @@ export function ProjectLibrary({ projects }: ProjectLibraryProps) {
           }
         />
       ) : viewMode === "cards" ? (
-        <ul className="grid grid-cols-1 gap-5 sm:grid-cols-2 lg:grid-cols-3">
+        <ul className="grid grid-cols-1 gap-5 sm:grid-cols-2 lg:grid-cols-3 xl:grid-cols-4">
           {visibleProjects.map((p) => (
             <li key={p.id}>
               <ProjectCard project={p} />

apps/web/components/ui/PageContainer.tsx

@@ -7,7 +7,7 @@ export type PageWidth = "prose" | "default" | "wide";
 const widths: Record<PageWidth, string> = {
   prose: "max-w-3xl",
   default: "max-w-5xl",
-  wide: "max-w-6xl",
+  wide: "max-w-7xl",
 };
 
 interface PageContainerProps {