Fix 'btn is not defined' JS error in Record meal modal

When the trigger JS was updated to use querySelectorAll('.roux-record-trigger')
for dual-button support, the variable was renamed from 'btn' to 'btns' but the
submit handler still referenced 'btn'. This commit:

- Adds activeBtn tracking: the button that opened the modal is stored in
  activeBtn via 'this' in the click handler
- Uses activeBtn.dataset.filename in the submit handler (with null guard)
- Removes the stale btn reference

src/Roux/Html.hs

@@ -654,9 +654,11 @@ cookLogJs =
         , "var dateInput = document.getElementById('roux-record-date');"
         , "var commentInput = document.getElementById('roux-record-comment');"
         , "var errorEl = document.getElementById('roux-record-error');"
+        , "var activeBtn = null;"
         , "if (!btns.length || !modal || !form) return;"
         , ""
         , "function showModal() {"
+        , "  activeBtn = this;"
         , "  dateInput.value = new Date().toISOString().slice(0,10);"
         , "  commentInput.value = '';"
         , "  errorEl.style.display = 'none';"
@@ -680,7 +682,8 @@ cookLogJs =
         , "  e.preventDefault();"
         , "  var date = dateInput.value;"
         , "  var comment = commentInput.value;"
-        , "  var filename = btn.dataset.filename;"
+        , "  if (!activeBtn) return;"
+        , "  var filename = activeBtn.dataset.filename;"
         , "  var body = 'cooked-date=' + encodeURIComponent(date);"
         , "  if (comment) body += '&comment=' + encodeURIComponent(comment);"
         , "  var res = await fetch('/cook-log/' + encodeURIComponent(filename), {"