Adds a comfortable/compact/dense/row toggle to every card grid in the
app. Each surface (LED devices, targets, automations, scenes, sources,
streams, dashboard subsections, etc.) remembers its mode independently.
Persistence mirrors dashboard-layout: localStorage cache for first paint,
debounced PUT to /api/v1/preferences/card-modes (new endpoint) for
cross-browser sync. Surface registry is open — any non-empty key
accepted server-side; modes validated against {comfortable, compact,
dense, row}.
CSS is token-driven: grid min-width and gap come from --card-grid-min /
--card-grid-gap / --card-grid-min-narrow / --card-grid-gap-narrow /
--templates-grid-min / --templates-grid-gap defined on :root, overridden
per [data-card-mode]. Dense/row also hide .mod-leds, collapse secondary
button labels, and tighten .mod-metrics; row collapses the grid to one
full-width column. Coexists with the existing per-section [data-density]
on the dashboard tab — different attribute, additive concern.
Toggle UI auto-mounts into every CardSection header (18+ surfaces) plus
the six dashboard subsections via post-render mount; teardown tracking
keeps the listener Set bounded across re-renders.
i18n: card_mode.{tooltip,comfortable,compact,dense,row} in en/ru/zh.
Tests: 9 new cases in tests/test_preferences_card_modes_api.py covering
defaults, round-trip, validation, open-registry keys, row mode, delete.
