Show the build in a footer, linked to its release or commit

The version moves from the header to a "Powered by Jarl" footer. A build
of a release tag (HEAD tagged v<VERSION>, no tracked changes; the release
workflow sets RELEASE_TAG) shows and links the release; any other build
shows and links its commit, marked -modified with uncommitted changes.
jarld version, jarl --version, /status and /auth/whoami report the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/version/version.go

@@ -1,7 +1,94 @@
 // Package version holds Jarl's version, set at build time with
 //
-//	go build -ldflags "-X jarl/internal/version.Version=1.2.3"
+//	go build -ldflags "-X jarl/internal/version.Version=1.2.3 -X jarl/internal/version.Tag=v1.2.3"
+//
+// The Makefile sets Version from the VERSION file, and Tag only when the
+// build is of that release: HEAD is the tag v<VERSION> and no tracked file
+// is changed. Other builds are identified by their commit, which Go
+// records from git by itself.
 package version
 
-// Version is the Jarl version.
+import (
+	"runtime/debug"
+	"sync"
+)
+
+// Version is the Jarl version (the VERSION file).
 var Version = "0.1.0-dev"
+
+// Tag is the release tag the build is of ("" when it isn't a release).
+var Tag = ""
+
+// Commit is the git commit the build is of. It is read from the build
+// information when not set at build time.
+var Commit = ""
+
+// Repo is Jarl's source repository.
+const Repo = "https://gitea.adbraun.lol/adam/jarl"
+
+// Build describes what a binary was built from.
+type Build struct {
+	Version  string `json:"version"`
+	Tag      string `json:"tag,omitempty"`
+	Commit   string `json:"commit,omitempty"`
+	Modified bool   `json:"modified,omitempty"` // built with uncommitted changes
+	// Label is the short form to show: the tag of a release, otherwise
+	// the abbreviated commit (or the version when the commit is unknown).
+	Label string `json:"label"`
+	// URL is the release or commit in the repository.
+	URL string `json:"url"`
+}
+
+var info = sync.OnceValue(func() Build {
+	commit, modified := Commit, false
+	if bi, ok := debug.ReadBuildInfo(); ok {
+		for _, s := range bi.Settings {
+			switch s.Key {
+			case "vcs.revision":
+				if commit == "" {
+					commit = s.Value
+				}
+			case "vcs.modified":
+				modified = s.Value == "true"
+			}
+		}
+	}
+	return describe(Version, Tag, commit, modified)
+})
+
+func describe(version, tag, commit string, modified bool) Build {
+	b := Build{Version: version, Tag: tag, Commit: commit, Modified: modified}
+	switch {
+	case tag != "":
+		b.Modified = false // the Makefile only sets Tag for clean trees
+		b.Label, b.URL = tag, Repo+"/releases/tag/"+tag
+	case commit != "":
+		b.Label, b.URL = short(commit), Repo+"/commit/"+commit
+		if modified {
+			b.Label += "-modified"
+		}
+	default:
+		b.Label, b.URL = version, Repo
+	}
+	return b
+}
+
+// Info describes this build.
+func Info() Build { return info() }
+
+// String is the version for logs and 'version' commands: "0.2.0" for a
+// release, otherwise the version and the commit ("0.2.0-dev (a1b2c3d4e5f6)").
+func String() string {
+	b := Info()
+	if b.Tag != "" || b.Commit == "" {
+		return b.Version
+	}
+	return b.Version + " (" + b.Label + ")"
+}
+
+func short(commit string) string {
+	if len(commit) > 12 {
+		return commit[:12]
+	}
+	return commit
+}