fix(assets): reference logo/favicon PNGs by URL instead of importing from public/

Importing images from public/ (`import X from '../../public/res/*.png'`) is
fragile under Vite with `publicDir: false` and can white-screen the dev app —
Vite tries to resolve the public/ path as a module. Switch the five logo/
favicon call sites to the repo's existing BASE_URL URL pattern
(`${trimTrailingSlash(import.meta.env.BASE_URL)}/public/res/...`), matching how
config.json, locales, and the Element Call widget are referenced. Resolves
identically in dev and the static-copied prod build (public/res -> dist/public/res).

Sounds under public/sound/ stay ESM-imported: that folder is not copied to
dist/ by vite-plugin-static-copy, so a URL reference would 404.

Also add a Local Development section to the README (no-backend model, npm ci /
npm start on :8080, which homeserver to log in against, OIDC-on-localhost note).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/pages/auth/AuthLayout.tsx

@@ -20,7 +20,7 @@ import {
 } from '../../hooks/useClientConfig';
 import { AsyncStatus, useAsyncCallback } from '../../hooks/useAsyncCallback';
 import { LOGIN_PATH, REGISTER_PATH, RESET_PASSWORD_PATH } from '../paths';
-import LotusLogo from '../../../../public/res/Lotus.png';
+import { trimTrailingSlash } from '../../utils/common';
 import { ServerPicker } from './ServerPicker';
 import { AutoDiscoveryAction, autoDiscovery } from '../../cs-api';
 import { SpecVersionsLoader } from '../../components/SpecVersionsLoader';
@@ -31,6 +31,8 @@ import { AuthFlowsProvider } from '../../hooks/useAuthFlows';
 import { AuthServerProvider } from '../../hooks/useAuthServer';
 import { tryDecodeURIComponent } from '../../utils/dom';
 
+const LotusLogo = `${trimTrailingSlash(import.meta.env.BASE_URL)}/public/res/Lotus.png`;
+
 const currentAuthPath = (pathname: string): string => {
   if (matchPath(LOGIN_PATH, pathname)) {
     return LOGIN_PATH;