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/client/WelcomePage.tsx

@@ -1,9 +1,11 @@
 import React from 'react';
 import { Box, Button, Icon, Icons, Text, config, toRem } from 'folds';
 import { Page, PageHero, PageHeroSection } from '../../components/page';
-import LotusLogo from '../../../../public/res/Lotus.png';
+import { trimTrailingSlash } from '../../utils/common';
 import pkg from '../../../../package.json';
 
+const LotusLogo = `${trimTrailingSlash(import.meta.env.BASE_URL)}/public/res/Lotus.png`;
+
 export function WelcomePage() {
   return (
     <Page>