feat: migrate all icons to Lucide React

Replace unicode characters, emoji, and inline SVGs with lucide-react
components across the entire design system and page layer. Update
tests to assert on SVG elements instead of text content.

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

package.json

@@ -35,6 +35,7 @@
     "test:e2e": "playwright test"
   },
   "dependencies": {
+    "lucide-react": "^1.7.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-router-dom": "^7.0.0"