Add integration icon

Home Assistant 2026.3+ shows custom integrations' own brand images from
custom_components/<domain>/brand/. Add a radiator with a forecast curve
(solid past, "now" marker, dashed prediction) as icon.png/icon@2x.png
(256/512 px, transparent) plus dark-theme variants, so the integration
is easy to spot in Devices & services. Older HA versions ignore the
folder and show the generic placeholder; the integration is unaffected.

scripts/make_brand_icons.py regenerates the images with Pillow from the
test requirements; the package check verifies sizes and transparency.
Listed under 0.2.3, which has not been deployed yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -25,6 +25,9 @@ A stale or failed room sensor no longer pauses the whole integration. Still
 - Status attributes `excluded_rooms` and `weight_coverage` (share of the total
   comfort weight covered by the rooms used), and a per-room `used` flag in
   `input_quality`.
+- Integration icon (a radiator with a forecast curve) in **Settings → Devices &
+  services**, with a dark-theme variant. Needs Home Assistant 2026.3 or newer;
+  older versions show the generic placeholder.
 
 ### Upgrade notes