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.

internal/web/service/server.go

@@ -117,7 +117,10 @@ type Status struct {
 
 // Release represents information about a software release from GitHub.
 type Release struct {
-	TagName string `json:"tag_name"` // The tag name of the release
+	TagName         string `json:"tag_name"`         // The tag name of the release
+	Body            string `json:"body"`             // The release notes; the dev channel reads its commit from here
+	TargetCommitish string `json:"target_commitish"` // The branch/commit the tag points at
+	Prerelease      bool   `json:"prerelease"`       // Whether this is a pre-release
 }
 
 // ServerService provides business logic for server monitoring and management.