perf(assets): convert images to webp and check asset paths

Convert 26 PNG/JPG assets to WebP: max 1600 px, q80, diagrams lossless.
src/assets shrinks from 8.3 MB to 4.2 MB.
Use me.jpg for og:image and twitter:image; some crawlers skip WebP.
Add scripts/check-assets.mjs to lint, so a wrong asset path fails lint,
not only at runtime.

README.md

@@ -90,6 +90,7 @@ Then open http://localhost:8080
 | `npm test` | Unit tests (Vitest + jsdom, no browser needed) |
 | `npm run lint` | ESLint over `src/**/*.ts` and `src/**/*.html` |
 | `npm run i18n:check` | Verifies `en.json` and `de.json` share the exact same keys |
+| `npm run assets:check` | Verifies every asset path referenced in `src/` exists (also runs in `npm run lint`) |
 
 ---
 
@@ -102,7 +103,7 @@ Triggered on every push to `main` and on pull requests targeting `main`.
 
 **Job 1 — `quality-check`** (runs on both pushes and PRs):
 
-1. Lint & type check — `npm run lint`
+1. Lint & type check — `npm run lint` (includes `npm run assets:check`)
 2. i18n key sync — `npm run i18n:check`
 3. Unit tests — `npm test`
 4. Production build — `npx ng build --configuration production`