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>

Makefile

@@ -14,7 +14,11 @@
 #                     (jarl-cli-<version>-<os>-<arch>[.exe])
 
 VERSION ?= $(shell cat VERSION 2>/dev/null || echo 0.0.0-dev)
-LDFLAGS := -s -w -X jarl/internal/version.Version=$(VERSION)
+# RELEASE_TAG marks a build of a release (the web UI then shows and links
+# the release rather than the commit): set when HEAD is tagged v$(VERSION)
+# and no tracked file is changed.
+RELEASE_TAG ?= $(shell t=v$(VERSION); git tag --points-at HEAD 2>/dev/null | grep -qx "$$t" && git diff --quiet HEAD 2>/dev/null && echo $$t)
+LDFLAGS := -s -w -X jarl/internal/version.Version=$(VERSION) -X jarl/internal/version.Tag=$(RELEASE_TAG)
 GOFLAGS := -trimpath
 export CGO_ENABLED := 0