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/oidc/oidcConfig.ts

@@ -1,8 +1,10 @@
 import type { OidcRegistrationClientMetadata } from 'matrix-js-sdk';
-import LotusLogo from '../../../../../public/res/Lotus.png';
+import { trimTrailingSlash } from '../../../utils/common';
 import { OIDC_CALLBACK_PATH } from '../../paths';
 import { getOriginBaseUrl, withOriginBaseUrl } from '../../pathUtils';
 
+const LotusLogo = `${trimTrailingSlash(import.meta.env.BASE_URL)}/public/res/Lotus.png`;
+
 /**
  * Absolute URL the OIDC provider redirects back to after authorization.
  *