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/dashboard.tmpl

@@ -0,0 +1,12 @@
+{{ define "content" }}
+<main class="container">
+  <aside class="sidebar">
+    <nav id="chanlist"></nav>
+  </aside>
+  <section class="chat">
+    <div id="tail"></div>
+  </section>
+</main>
+{{ end }}
+{{ template "layout.tmpl" . }}
+