feat: add fullscreen toggle button to dashboard

Added a fullscreen button (⛶) next to the last-update timestamp.
Clicking it toggles browser fullscreen mode using the Fullscreen API.

wwwroot/index.ui

@@ -37,6 +37,27 @@
                         "text": "最后更新: {{get_today_usage(request) and request._run_ns.curDateString() or ''}}",
                         "fontSize": "13px"
                     }
+                },
+                {
+                    "widgettype": "Button",
+                    "options": {
+                        "label": "⛶",
+                        "border": "none",
+                        "borderRadius": "6px",
+                        "padding": "4px 12px",
+                        "fontSize": "16px",
+                        "marginLeft": "12px",
+                        "cursor": "pointer"
+                    },
+                    "binds": [
+                        {
+                            "wid": "self",
+                            "event": "click",
+                            "actiontype": "script",
+                            "target": "self",
+                            "script": "if (!document.fullscreenElement) { document.documentElement.requestFullscreen(); } else { document.exitFullscreen(); }"
+                        }
+                    ]
                 }
             ]
         }