dashboard: surface the hive's hyperhive rev on the H0M3 start page

Adds hyperhive_rev to the dashboard's /api/state StateSnapshot,
resolved via the same current_flake_rev helper get_agent_meta's
per-agent hyperhive_rev already uses. home.js renders it next to the
existing hive-identity line, truncated to the last 12 chars with the
full value in title=, hidden when the flake ref isn't a local path pin.

Requested by annika (infra.run) via dmatrix, hyperhive#2854.

frontend/packages/dashboard/src/index.html

@@ -30,6 +30,7 @@
   <header class="home-header">
     <p class="banner-thin" id="hive-identity" hidden></p>
     <pre class="banner">░▒▓█▓▒░  ░▒▓█▓▒░  H Y P E R H I V E  ·  H0M3  ░▒▓█▓▒░  ░▒▓█▓▒░</pre>
+    <p class="banner-thin" id="hive-rev" hidden></p>
   </header>
 
   <main class="home-main">