refactor: standalone dashboard folders with index.html, zip-based import/export (.tsd/.tss/.tsp)

- Each dashboard is now a self-contained folder: manifest.json + index.html
- Auto-scan dashboards/ and data/dashboards/ on startup
- Export: .tsd (dashboards), .tss (scenes), .tsp (game plugins) - all zip format
- Dashboard index.html is complete standalone page (WS + data binding)
- Aspect ratio constraints handled in each dashboard's own JS
- Removed template-based dashboard rendering in favor of static serve
- Import via file upload endpoints, export via direct file download

dashboards/speedometer/manifest.json

@@ -0,0 +1,15 @@
+{
+    "id": "speedometer",
+    "name": "极速仪表 - 速度表",
+    "category": "speed",
+    "description": "大字号实时速度显示，支持km/h和mph，适合横屏全屏显示",
+    "author": "Yei.J. (AskaEth)",
+    "version": "1.0.0",
+    "config": {
+        "icon": "🏎️",
+        "unit": "kmh",
+        "show_unit": true,
+        "aspect_ratio": "auto",
+        "render_mode": "contain"
+    }
+}