README screenshots from seeded demo data

## Context

`README.md` has a screenshots section with the image block commented out, and `DEVELOPING.md` documents `KALENDEE_SEED_DEMO=true` demo data (see `DemoSeeder.kt`). This issue captures the product screens from a seeded run, stores them under `docs/screenshots/`, and uncomments the README block so the repo presents the real UI. No mockups or fabricated data — only seeded demo content.

## Tasks

- [ ] Run the server with `KALENDEE_SEED_DEMO=true` and a fixed theme/viewport for reproducible captures
- [ ] Capture: week view, event dialog, notifications, settings, public calendar
- [ ] Store optimized PNGs under `docs/screenshots/` with stable file names
- [ ] Uncomment/extend the README image block (`README.md:18-20`)
- [ ] Update the `DEVELOPING.md` "Demo data and screenshots" section with the exact capture workflow
- [ ] Confirm no real personal data, tokens, or emails appear in captures

## References

- `README.md:14-20` — Screenshots section and commented block
- `DEVELOPING.md` — Demo data and screenshots
- `server/src/main/kotlin/dev/kolektiv/kalendee/demo/DemoSeeder.kt`
- `server/src/main/resources/application.conf:60-61` — `seedDemo` / `KALENDEE_SEED_DEMO`
- `docs/screenshots/`

## Dependencies

- None; can be done immediately. Re-capture when the UI changes materially (e.g. after #2 adds the connections tab).