Line tree: gold outline follows the chain's BuyerCounted events, plus a legend

The Overview tree painted gold on the first N chips (N = buyer count) instead of
the members the contract actually counted, so a non-buyer listed first showed as
qualified (Hugh's line, 2026-09-12). /api/my/line now flags each member with
qualified from BuyerCounted events for the account's ids; the tree uses that flag
and shows a legend for gold / plain / open chips.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/my.html

@@ -234,6 +234,7 @@
         <div class="card" id="lineTreeCard">
           <div class="card-head"><h3>Your line at a glance</h3><span class="sub" id="treeSub"></span></div>
           <div id="lineTree" class="line-tree"><p class="muted small">Loading your line…</p></div>
+          <p class="small muted" id="treeLegend" style="margin:10px 0 0;line-height:2"><span class="lt-node qualified" style="font-size:11px;padding:2px 8px">name</span> qualifying buyer: bought a $20+ package, counts toward your levels &nbsp;·&nbsp; <span class="lt-node" style="font-size:11px;padding:2px 8px">name</span> joined, no $20+ buy yet &nbsp;·&nbsp; <span class="lt-node open" style="font-size:11px;padding:2px 8px">+ open</span> spot still open</p>
         </div>
         <div class="card" id="badgeCard" hidden>
           <div class="card-head"><h3>Your achievements</h3><span class="sub">unlock as you build</span></div>