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>

README.md

@@ -167,6 +167,23 @@ The source code lives in `/root/code/cinny`. All changes should be made on the `
 
 See [LOTUS_FEATURES.md](LOTUS_FEATURES.md) for the full feature changelog and [LOTUS_TODO.md](LOTUS_TODO.md) for the work backlog.
 
+### Local Development
+
+Lotus Chat is a **pure client — there is no backend of its own to run.** It talks directly to a Matrix homeserver (Synapse) over HTTPS, so the only thing you run locally is the Vite dev server; it connects to a real homeserver for all data. If you were looking for "the backend to pair with it," there isn't one — that's the homeserver.
+
+**Prerequisites:** Node 20+ (CI builds on Node 24) and npm.
+
+```bash
+npm ci        # deps; @lotusguild/* come from our Gitea npm registry (public read — no auth/token needed)
+npm start     # Vite dev server → http://localhost:8080
+```
+
+The dev server defaults to **port 8080** (`vite.config.js`); if 8080 is already in use it falls through to 8081+, so check the "Local:" URL Vite prints on startup. If it boots but the page renders blank, it's almost always a failed module/asset resolution, not a "missing backend" — open the devtools console and read the first error.
+
+**Which homeserver / logging in:** `config.json` sets `defaultHomeserver: 0` → `matrix.lotusguild.org`, so you sign in with your normal `@you:matrix.lotusguild.org` account. That homeserver is **live production** — anything you send is real, so keep test traffic to a DM with yourself or a throwaway room. To develop fully isolated instead, point `config.json` at a throwaway `matrix.org` account (already in `homeserverList`) or a local Synapse.
+
+- **SSO / OIDC works from localhost.** Login goes through Authelia via OIDC dynamic registration; the provider redirects back to `http://localhost:8080/…` and the client registers that redirect on the fly, so no server-side allow-listing is needed. After the callback you may see a `GET …/_matrix/media/v1/thumbnail/… 404` — that's just a missing avatar thumbnail, **not** a login failure.
+
 ### 🔱 Element Call fork ("Lotus Call") — LIVE
 
 Voice/video channels embed **Element Call**, which is now our **self-built fork**