chore: replace biome with prettier

Remove @biomejs/biome, set indent size to 4 and add VS Code format-on-save with the Prettier extension.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016GHq7GcQtB91bnq3832H76

.vscode/extensions.json

@@ -1,4 +1,4 @@
 {
-  // For more information, visit: https://go.microsoft.com/fwlink/?linkid=827846
-  "recommendations": ["angular.ng-template"]
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=827846
+    "recommendations": ["angular.ng-template", "esbenp.prettier-vscode"]
 }

.vscode/settings.json

@@ -0,0 +1,4 @@
+{
+    "editor.defaultFormatter": "esbenp.prettier-vscode",
+    "editor.formatOnSave": true
+}