Add --demo mode with fake router data + README screenshots

[vpadman1]

Summary

Running the dashboard against a real router means any screenshot for the README would leak your SSID, neighbours' SSIDs, connected-device MACs, and hostnames. This PR adds a `--demo` mode that runs against hardcoded plausible data so the tool can be demoed, screenshotted, and tried by anyone — including people who don't own a compatible router yet.

What's in it

• `drivers/demo.py` — `DemoDriver` with two bands (`MyHomeWiFi` / `MyHomeWiFi-5G`) and four fake clients. `set_channel` mutates internal state so pressing C in the TUI reflects on the next refresh — the demo feels live, not static.
• `devices/demo.yaml` — registers the driver so it also shows up in `--list-devices` and can be invoked via `--device demo`.
• `wifi_scanner.demo_scan_networks()` — 10-network fake scan crafted so the RSSI-weighted recommender has a clear winner on each band (channel 11 on 2.4 GHz, channel 149 on 5 GHz).
• `WifiDashboard` now accepts an optional `scan_fn` kwarg so `--demo` can wire in the canned scan instead of calling CoreWLAN.
• `main.py` — new `--demo` flag that bypasses keyring, password, and login entirely. Also adds a "Try `--demo` first" hint to the no-password error message so new users have an easy path in.
• `README.md` — new "Try it in 30 seconds (no router needed)" section at the top, with both screenshots embedded.
• `docs/screenshot.png`, `docs/screenshot-apply.png` — real captures of the `--demo` dashboard (not leaking any real network data).
• 9 new tests covering DemoDriver correctness, get_all_data enrichment, set_channel state updates, and scan data shape.

Why this matters for adoption

From the discoverability audit: a CLI tool without a screenshot in the README gets a high bounce rate, and Textual dashboards are visually compelling. `--demo` also makes the "hey, try this out" barrier near-zero — clone, one command, see the thing work.

Test plan

• `uv run pytest` — 97 passed (88 existing + 9 new)
• `uvx ruff check .` — clean
• Smoke test: `WifiDashboard(client=DemoDriver(), scan_fn=demo_scan_networks).compose()` builds widgets without error
• Manual run: `uv run python main.py --demo` launches cleanly; R/S/C/Q keybindings all behave
• Screenshots taken from the real `--demo` output
• CI green