legal: close AGPL packaging gaps

Name the copyright holder, ship NOTICE with the image, serve the licence
from the panel footer on every page, and record the vendored htmx licence.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/templates.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"html/template"
 	"net/http"
+
+	"github.com/mixeme/selfpost/internal/legal"
 )
 
 // templates holds the parsed page and fragment templates. Each page is parsed
@@ -84,13 +86,15 @@ func (s *Server) render(w http.ResponseWriter, status int, page string, data any
 	// The layout's navigation compares .Active against each item, so the key
 	// must exist on every authenticated page. Defaulting it here keeps a page
 	// that forgets it from failing to render — it simply highlights nothing.
-	// .Version, shown in the layout's footer, is supplied the same way: it is
-	// the same value on every page, so no handler should have to pass it.
+	// Footer fields (.Version, .Copyright, .SourceURL) are the same on every
+	// page, so no handler should have to pass them.
 	if m, ok := data.(map[string]any); ok {
 		if _, has := m["Active"]; !has {
 			m["Active"] = ""
 		}
 		m["Version"] = s.cfg.Version
+		m["Copyright"] = legal.CopyrightLine
+		m["SourceURL"] = legal.SourceURL
 	}
 	var buf bytes.Buffer
 	if err := tmpl.ExecuteTemplate(&buf, "layout.html", data); err != nil {