chore: replace Unicode/emoji icons with Lucide React

Adds lucide-react and replaces all HTML entity and emoji icons across
the UI with proper SVG icon components. Tree-shaken — only imported
icons are bundled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/DiagramNode.tsx

@@ -133,31 +133,19 @@ export function DiagramNode({
       {isCompleted && (
         <>
           <circle cx={w - 10} cy={TOP_BAR_HEIGHT + 8} r={6} fill="#3D7C47" />
-          <text
-            x={w - 10}
-            y={TOP_BAR_HEIGHT + 11}
-            textAnchor="middle"
-            fill="white"
-            fontSize={9}
-            fontWeight={700}
-          >
-            &#x2713;
-          </text>
+          <path
+            d={`M${w - 13} ${TOP_BAR_HEIGHT + 8} l2 2 4-4`}
+            fill="none" stroke="white" strokeWidth={1.5} strokeLinecap="round" strokeLinejoin="round"
+          />
         </>
       )}
       {isFailed && (
         <>
           <circle cx={w - 10} cy={TOP_BAR_HEIGHT + 8} r={6} fill="#C0392B" />
-          <text
-            x={w - 10}
-            y={TOP_BAR_HEIGHT + 11}
-            textAnchor="middle"
-            fill="white"
-            fontSize={9}
-            fontWeight={700}
-          >
-            !
-          </text>
+          <path
+            d={`M${w - 10} ${TOP_BAR_HEIGHT + 5} v4 M${w - 10} ${TOP_BAR_HEIGHT + 10.5} v0.5`}
+            fill="none" stroke="white" strokeWidth={1.5} strokeLinecap="round"
+          />
         </>
       )}
 
@@ -177,15 +165,13 @@ export function DiagramNode({
 
       {/* Sub-route failure: drill-down arrow at bottom-left */}
       {isFailed && executionState?.subRouteFailed && (
-        <text
-          x={6}
-          y={h - 4}
-          fill="#C0392B"
-          fontSize={11}
-          fontWeight={700}
-        >
-          &#x21B3;
-        </text>
+        <g transform={`translate(4, ${h - 14})`}>
+          <path
+            d="M2 2 v5 a3 3 0 003 3 h5"
+            fill="none" stroke="#C0392B" strokeWidth={1.5} strokeLinecap="round" strokeLinejoin="round"
+          />
+          <path d="M8 8 l2 2 -2 2" fill="none" stroke="#C0392B" strokeWidth={1.5} strokeLinecap="round" strokeLinejoin="round" />
+        </g>
       )}
     </g>
   );