fix(local-tool): SPA fallback 改用 Next.js dynamic route shell HTML

根因：點模型卡片 → /models/yolov5-face-detection → server SPA fallback
固定回傳根路徑 /index.html（儀表板） → Next.js CSR router 初始化時
pathname 對不上 → 使用者被跳回儀表板。

修法：spaFallback handler 改成三層 fallback：
1. 精確檔案（/models/index.html 等）
2. Next.js dynamic route shell（把最後一段替換為 _ → /models/_/index.html）
   這是 generateStaticParams 產的 placeholder 頁面，Next.js CSR 會從 URL
   讀真正的 param 值
3. 根目錄 /index.html（最終 fallback）

這修好了：
- 模型詳情頁 /models/:id 不再跳回儀表板
- 裝置詳情頁 /devices/:id 同理
- 工作區裝置頁 /workspace/:deviceId 同理
- Sidebar active 狀態也會正確（因為 pathname 匹配了）

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

local-tool/server/internal/api/router.go

@@ -144,7 +144,17 @@ func broadcasterLogger(b *logger.Broadcaster) gin.HandlerFunc {
 }
 
 // spaFallback tries to serve the exact file from the embedded FS.
-// If the file doesn't exist, it serves index.html for client-side routing.
+// If the file doesn't exist, it finds the best matching route shell HTML
+// for Next.js static export client-side routing.
+//
+// Next.js static export with generateStaticParams creates:
+//   /models/index.html       — static page
+//   /models/_/index.html     — dynamic route shell (placeholder param '_')
+//
+// For a request like /models/yolov5-face-detection:
+//  1. Try exact file → not found
+//  2. Try /models/_/index.html → found → serve it (Next.js CSR picks up real param from URL)
+//  3. Fall back to /index.html (root)
 func spaFallback(staticFS http.FileSystem) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		path := c.Request.URL.Path
@@ -155,15 +165,31 @@ func spaFallback(staticFS http.FileSystem) gin.HandlerFunc {
 			return
 		}
 
-		// Try to serve the exact file (e.g., /models/index.html)
-		f, err := staticFS.Open(path)
-		if err == nil {
+		// Try to serve the exact file
+		if f, err := staticFS.Open(path); err == nil {
 			f.Close()
 			http.FileServer(staticFS).ServeHTTP(c.Writer, c.Request)
 			return
 		}
 
-		// Fall back to root index.html for SPA routing
+		// Try Next.js dynamic route shell: replace last path segment with '_'
+		// e.g. /models/yolov5 → /models/_/index.html
+		//      /devices/kl520-0 → /devices/_/index.html
+		//      /workspace/kl520-0 → /workspace/_/index.html
+		segments := strings.Split(strings.TrimRight(path, "/"), "/")
+		if len(segments) >= 2 {
+			segments[len(segments)-1] = "_"
+			shellPath := strings.Join(segments, "/") + "/index.html"
+			if f, err := staticFS.Open(shellPath); err == nil {
+				defer f.Close()
+				c.Header("Content-Type", "text/html; charset=utf-8")
+				c.Status(http.StatusOK)
+				_, _ = io.Copy(c.Writer, f)
+				return
+			}
+		}
+
+		// Final fallback: root index.html
 		index, err := staticFS.Open("/index.html")
 		if err != nil {
 			c.Status(http.StatusInternalServerError)