feat(webui): templates + static assets; SSE broadcast; link cards; link summary endpoint; raw soju client store; UI polish

- Switch UI to Go html/templates and embedded /static (Pico.css-compatible)

- Add Server.Broadcast and normalize SSE channel keys to lowercase

- Implement /api/linkcard (OG/Twitter) and /api/linksummary (24h cache)

- Wire Store into raw soju client for CHATHISTORY LATEST fallback

internal/httpapi/templates.go

@@ -0,0 +1,47 @@
+package httpapi
+
+import (
+	"embed"
+	"html/template"
+	"net/http"
+	"path"
+	"strings"
+	"sync"
+)
+
+//go:embed templates/*.tmpl
+var uiFS embed.FS
+
+//go:embed static/*
+var staticFS embed.FS
+
+var (
+	tplOnce sync.Once
+	tpl     *template.Template
+)
+
+func (s *Server) parseTemplatesOnce() {
+	tplOnce.Do(func() {
+		// Parse all templates under templates/
+		t := template.New("base").Funcs(template.FuncMap{})
+		t = template.Must(t.ParseFS(uiFS, "templates/*.tmpl"))
+		tpl = t
+	})
+}
+
+func (s *Server) render(w http.ResponseWriter, name string, data map[string]any) {
+	s.parseTemplatesOnce()
+	if data == nil {
+		data = map[string]any{}
+	}
+	data["Version"] = s.Version
+	data["Commit"] = s.Commit
+	base := strings.TrimSuffix(path.Base(name), path.Ext(name))
+	if base == "dashboard" {
+		data["Title"] = "sojuboy"
+	} else {
+		data["Title"] = strings.Title(base)
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_ = tpl.ExecuteTemplate(w, name, data)
+}