add sidebar footer RSS readout (polls /memory every 30s)

Footer now shows "<bun-rss> · <tab-count>" sourced from the /memory
endpoint, polled every 30s. Color thresholds: orange warn at 2 GB Bun
RSS or 50 tabs; red bad at 8 GB or 200 tabs (matches the tab-guardrail
threshold landing in a later commit). The footer gives the user an
early signal that the cliff is forming, instead of only learning when
the OS OOM-kills the process.

Backoff per Codex's flag: if a poll takes > 2s response time the
sidebar drops to a 5-minute cadence until the next successful fast
poll. The diagnostic shouldn't add load to a browser that's already
unhealthy.

Start/stop is wired to the existing setServerInfo() hook so the timer
only runs while the sidebar is connected to a server.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

extension/sidepanel.html

@@ -166,6 +166,7 @@
       <button class="footer-btn" id="reload-sidebar" title="Reload sidebar">reload</button>
     </div>
     <div class="footer-right">
+      <span class="footer-mem" id="footer-mem" title="Process memory + tab count from $B memory (polled every 30s, paused if slow)"></span>
       <span class="dot" id="footer-dot"></span>
       <span class="footer-port" id="footer-port" title="Click to change port"></span>
       <input type="text" class="port-input" id="port-input" placeholder="34567" autocomplete="off" style="display:none">