feat: init Astro blog with wikilinks, KaTeX and local search

- Astro 5 static blog, zero runtime JS
- Obsidian wikilink remark plugin (slug matches Astro content collection)
- KaTeX math rendering, local search, TOC, archives, tags
- 5 posts on math/physics/ML topics
- deploy scripts for nginx + certbot + rsync

deploy/nginx-serve.conf

@@ -0,0 +1,47 @@
+# Yukun's Blog · nginx 共享服务配置片段
+# 由 deploy/deploy.sh setup-nginx 上传到 /etc/nginx/snippets/yukun.conf
+# 80 与 443 段都 include 此文件，避免重复
+
+root /var/www/yukun;
+index index.html;
+
+# gzip 压缩
+gzip on;
+gzip_vary on;
+gzip_min_length 1024;
+gzip_comp_level 6;
+gzip_types
+    text/plain
+    text/css
+    text/xml
+    text/javascript
+    application/javascript
+    application/x-javascript
+    application/json
+    application/xml
+    application/xml+rss
+    image/svg+xml;
+
+# 静态资源长缓存（Astro 构建产物带 hash，可放心强缓存）
+location ~* \.(?:css|js|woff2?|ttf|otf|svg|png|jpg|jpeg|gif|webp|ico|avif)$ {
+    expires 30d;
+    add_header Cache-Control "public, immutable";
+    access_log off;
+}
+
+# sitemap 不缓存
+location = /sitemap-index.xml { add_header Cache-Control "no-cache"; }
+location ~ ^/sitemap.* { add_header Cache-Control "no-cache"; }
+
+# Astro 目录式 URL，确保 index.html 能解析
+location / {
+    try_files $uri $uri/ $uri.html =404;
+}
+
+# 自定义 404
+error_page 404 /404.html;
+
+# 安全头
+add_header X-Content-Type-Options "nosniff" always;
+add_header X-Frame-Options "SAMEORIGIN" always;
+add_header Referrer-Policy "strict-origin-when-cross-origin" always;