feat(update): add rolling dev update channel for per-commit builds

Adds an opt-in Dev channel so panels running CI per-commit builds can self-update to the latest commit, mirroring the stable online-update flow.

CI publishes/overwrites a single fixed-tag pre-release (dev-latest), force-moved to the newest main commit and marked --latest=false so releases/latest stays the stable tag. Builds stamp the short commit via -ldflags; the panel compares the running commit to the dev release commit to detect an update, and update.sh honors XUI_UPDATE_TAG to install from that tag. Linux/systemd only.

frontend/public/openapi.json

@@ -4240,6 +4240,49 @@
         }
       }
     },
+    "/panel/api/server/setUpdateChannel": {
+      "post": {
+        "tags": [
+          "Server"
+        ],
+        "summary": "Toggle the panel update channel between stable and the rolling per-commit dev release. Only effective on dev builds.",
+        "operationId": "post_panel_api_server_setUpdateChannel",
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "type": "object"
+              },
+              "example": {
+                "dev": true
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "Successful response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "success": {
+                      "type": "boolean"
+                    },
+                    "msg": {
+                      "type": "string"
+                    },
+                    "obj": {}
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/panel/api/server/updateGeofile": {
       "post": {
         "tags": [