Fix SPA routing: fallback to index.html for Vue Router paths

Direct navigation to /bugs, /oils, /manage etc. returned 404 because
FastAPI's StaticFiles only served index.html for /. Now all non-API,
non-asset routes return index.html so Vue Router handles client-side routing.

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

backend/main.py

@@ -1494,4 +1494,18 @@ def startup():
         seed_defaults(data["oils_meta"], data["recipes"])
 
     if os.path.isdir(FRONTEND_DIR):
-        app.mount("/", StaticFiles(directory=FRONTEND_DIR, html=True), name="frontend")
+        # Serve static assets (js/css/images) directly
+        app.mount("/assets", StaticFiles(directory=os.path.join(FRONTEND_DIR, "assets")), name="assets")
+        app.mount("/public", StaticFiles(directory=FRONTEND_DIR), name="public")
+
+        # SPA fallback: any non-API, non-asset route returns index.html
+        from fastapi.responses import FileResponse
+
+        @app.get("/{path:path}")
+        async def spa_fallback(path: str):
+            # Serve actual files if they exist (favicon, icons, etc.)
+            file_path = os.path.join(FRONTEND_DIR, path)
+            if os.path.isfile(file_path):
+                return FileResponse(file_path)
+            # Otherwise return index.html for Vue Router
+            return FileResponse(os.path.join(FRONTEND_DIR, "index.html"))