Add a brand icon for the integration (#447)

* Add a brand icon for the integration

Serve a brand icon via the Brands Proxy API by shipping it in
custom_components/victron_mqtt/brand/ (icon.png 256x256, icon@2x.png 512x512).
Original, neutral energy/MQTT motif — not the Victron trademark.

* brand: use official Victron icons from home-assistant/brands

Replace AI-generated icons with the existing official Victron artwork
from home-assistant/brands/custom_integrations/victron_mqtt.

Since HA 2026.3 the Brands Proxy API serves icons from the integration's
local brand/ folder first, falling back to the central brands CDN.
Shipping the icons locally fixes the empty-placeholder view in the HA
integration list without changing the visual branding.

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Co-authored-by: pos-ei-don <philipp.wacker@ibf-solutions.com>

Author: pos-ei-don