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/summarizer.tmpl

@@ -0,0 +1,14 @@
+{{ define "content" }}
+<main class="container">
+  <article>
+    <h3>On-demand summarization</h3>
+    <label>Channel<select id="channel"></select></label>
+    <label>Window<input id="window" value="6h"></label>
+    <label><input type="checkbox" id="push"> Send via Pushover</label>
+    <button id="btn" onclick="summarize()">Summarize</button>
+    <pre id="out"></pre>
+  </article>
+</main>
+{{ end }}
+{{ template "layout.tmpl" . }}
+