Add dev/demo login selector for quick user switching

When AUTH_DISABLED=true, the app now shows a login selector page that
lists staff members and clients from the database. Selecting a user
sets a localStorage-based session and sends X-Dev-User-Id header on
all API requests. A persistent bottom bar shows the active persona
with a "Switch user" link.

- API: /api/dev/config (public) and /api/dev/users (auth-disabled only)
- API: auth middleware reads X-Dev-User-Id header when auth is disabled
- Frontend: DevLoginSelector page, DevSessionIndicator bar
- Frontend: fetch interceptor injects X-Dev-User-Id on /api/* calls
- Tests: 7 passing (5 nav + 2 dev login)

Closes #60

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/main.tsx

@@ -2,8 +2,11 @@ import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
 import { App } from "./App.js";
+import { installDevFetchInterceptor } from "./lib/devFetch.js";
 import "./index.css";
 
+installDevFetchInterceptor();
+
 const root = document.getElementById("root");
 if (!root) throw new Error("Root element not found");