docs: update dark mode plan — CartoDB tiles replace Stadia (no key required)

docs/superpowers/plans/2026-06-19-dark-mode.md

@@ -15,8 +15,9 @@
 - Existing token names in `tokens.css` must not change — only values swap
 - No new npm/JS dependencies
 - `make test-ui` must pass after every task (pre-existing P2 FilePond failure is acceptable)
-- Stadia Maps tile URL: `https://tiles.stadiamaps.com/tiles/alidade_smooth_dark/{z}/{x}/{y}{r}.png`
-- Stadia attribution (exact): `© <a href="https://stadiamaps.com/">Stadia Maps</a> © <a href="https://openmaptiles.org/">OpenMapTiles</a> © <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors`
+- Map tile URL: `https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png` (CartoDB — no API key required)
+- CartoDB attribution (exact): `© <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors © <a href="https://carto.com/attributions">CARTO</a>`
+- Note: Stadia Maps requires an API key even for local dev — CartoDB dark_all is the keyless alternative
 
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