Regenerate toy tutorial screenshots for kW units

[Flix6x]

Companion to FlexMeasures/flexmeasures#2223, which switches the toy tutorial to kW-scale power sensors and EUR/kWh day-ahead prices. That PR's "Further Improvements" asks for the external UI screenshots to be regenerated here.

How these were made

All five toy tutorial parts were run in order against a fresh database on the feat/change-toy-tutorial-to-kw-units branch, and each chart was exported with the chart's own Save as PNG action in the UI (not a browser screenshot), so the framing matches how the existing charts were produced.

The run reproduces the numbers the tutorial docs claim on that branch: headroom bottoms out at 280 kW around noon, and the process costs come to 9.60 / 4.30 / 5.40 EUR for the inflexible / breakable / shiftable policies.

Commit 1 — charts affected by the unit change

Power axes go from MW (±0.5) to kW (±500), prices from EUR/MWh (4–18) to EUR/kWh (0.004–0.018), and the process cost reports from hundreds of EUR to fractions of a EUR:

asset-view-process, asset-view-with-solar, asset-view-without-solar, sensor-data-breakable, sensor-data-charging, sensor-data-charging-with-solar, sensor-data-headroom, sensor-data-headroom-nopv, sensor-data-headroom-pv, sensor-data-inflexible, sensor-data-multiasset-negativeprices, sensor-data-prices, sensor-data-production, sensor-data-shiftable.

Two incidental differences from the old set: the charts now carry the plot titles the UI renders today ("Prices", "Power flows"), and the asset-page charts include the price panel that the old images were cropped to exclude.

Commit 2 — the other toy tutorial images

Only toy-tutorial-building-flex-context.png changes: the consumption-price sensor now reads Unit: EUR/kWh.

The rest were re-rendered and deliberately left alone:

Image	Why not updated
toy-tutorial-battery-flex-model	Values are unchanged by #2223 (450 kWh, 500 kVA, 90%)
toy-tutorial-site-structure	Asset structure is unaffected
asset-view-dashboard	Needs a Mapbox token to render the map; my instance renders a blank grey map
screenshot_building_status	Needs Redis for the jobs panel, and shows a red lamp for the price sensor outside the Docker tutorial stack

Note that all four (plus the flex-model one) are also stale in other ways relative to the current UI — the starter template assets now shift the toy sensor IDs, so the IDs in the images and in the tutorial docs no longer line up. That's worth a separate pass and is not caused by #2223.

[BelhsanHmida]

I verified the screenshots. They look good overall; I only found two screenshots with stale asset IDs in the browser URL. I refreshed those so the site structure screenshot now shows /assets/5/context and the battery flex-model screenshot now shows /assets/6/properties, matching the updated tutorial docs.