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_test.go

@@ -0,0 +1,22 @@
+package version
+
+import "testing"
+
+func TestDescribe(t *testing.T) {
+	const c = "4d00be5e1f0a9b8c7d6e5f4a3b2c1d0e9f8a7b6c"
+	for _, tc := range []struct {
+		tag, commit string
+		modified    bool
+		label, url  string
+	}{
+		{"v0.1.0", c, true, "v0.1.0", Repo + "/releases/tag/v0.1.0"},
+		{"", c, false, "4d00be5e1f0a", Repo + "/commit/" + c},
+		{"", c, true, "4d00be5e1f0a-modified", Repo + "/commit/" + c},
+		{"", "", false, "0.1.0", Repo},
+	} {
+		b := describe("0.1.0", tc.tag, tc.commit, tc.modified)
+		if b.Label != tc.label || b.URL != tc.url {
+			t.Errorf("describe(%q, %q, %v) = %q %q, want %q %q", tc.tag, tc.commit, tc.modified, b.Label, b.URL, tc.label, tc.url)
+		}
+	}
+}