fix: surface full sitespeed.io error output + robust binary resolution

- runner.js: auto-detects sitespeed.io via which/candidates instead of
  hardcoded path; collects all stderr and appends last 20 lines to the
  error so the real failure is visible in the UI log
- running.pug: render multi-line error messages line-by-line

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/running.pug

@@ -65,9 +65,14 @@ block content
       badge.className = 'text-xs px-3 py-1 rounded-full bg-red-100 text-red-800 font-semibold';
       try {
         const d = JSON.parse(e.data);
-        const el = document.createElement('div');
-        el.className = 'text-red-400 log-line';
-        el.textContent = '[ERROR] ' + d.message;
-        logBox.appendChild(el);
+        // Render each line of the error message separately so multi-line errors are readable
+        const lines = d.message.split('\n');
+        for (const line of lines) {
+          const el = document.createElement('div');
+          el.className = 'text-red-400 log-line';
+          el.textContent = line;
+          logBox.appendChild(el);
+        }
+        logBox.scrollTop = logBox.scrollHeight;
       } catch {}
     });