feat: remove Reflex, archive old app, update docs for Dash migration (Task 5.4)

- Remove reflex dependency from pyproject.toml
- Move pathways_app/ and rxconfig.py to archive/
- Update CLAUDE.md: Dash app structure, callback chain, run command
- All completion criteria validated (10/10 pass)

archive/rxconfig.py

@@ -0,0 +1,14 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent / "src"))
+
+import reflex as rx
+
+config = rx.Config(
+    app_name="pathways_app",
+    plugins=[
+        rx.plugins.SitemapPlugin(),
+        rx.plugins.TailwindV4Plugin(),
+    ]
+)