Reduce navigation flash with instant theme background and view transitions.

Co-authored-by: Cursor <cursoragent@cursor.com>

static/css/tech.css

@@ -1,8 +1,25 @@
 /* Copyright (c) 2025-2026 马建军. All rights reserved. 专有软件，详见 LICENSE.zh-CN.txt */
 /* 科技感增强层 — 与 base.html 变量配合 */
 
+@view-transition {
+    navigation: auto;
+}
+
+::view-transition-old(app-main),
+::view-transition-new(app-main) {
+    animation-duration: .18s;
+}
+
+::view-transition-old(app-tech-bg),
+::view-transition-new(app-tech-bg),
+::view-transition-old(site-header),
+::view-transition-new(site-header) {
+    animation: none;
+}
+
 .tech-bg{
     position:fixed;inset:0;z-index:0;pointer-events:none;overflow:hidden;
+    view-transition-name:app-tech-bg;
 }
 .tech-grid{
     position:absolute;inset:0;
@@ -59,6 +76,7 @@
     border-bottom:1px solid var(--border-header);
     background:transparent;
     backdrop-filter:none;
+    view-transition-name:site-header;
 }
 .site-header::after{
     content:"";display:block;height:1px;margin-top:-1px;