Chart builder: drop unparseable expressions mid-edit, polite glyphed inline feedback

src/app/components/ChartBuilderModal.module.css

@@ -234,11 +234,21 @@
 
 .exprError,
 .exprWarn {
+  display: flex;
+  align-items: flex-start;
+  gap: var(--space-2);
   margin: 0;
   font-size: 11px;
   line-height: 1.4;
 }
 
+/* Severity reads from the glyph shape + colour, not colour alone (arch 10 §3); the
+   icon inherits the line's colour via currentColor (round error / triangle warning). */
+.exprFeedbackIcon {
+  flex: none;
+  margin-top: 1px;
+}
+
 .exprError {
   color: var(--support-error);
 }