No operator preference layer (units, theme, landing desk, chart range persistence)

Only three ad-hoc browser keys existed (dsc_chart_hours in sessionStorage, dsc-hub-snooze:*, dsc_compose_tent). Pass S1 (2026-09-07, feat/dashboard-v2, uncommitted): lib/preferences.ts store (dsc.prefs.v1), usePreference hooks, Preferences section with Appearance / Units (metric only) / Home / Charts cards; grid wash, motion, depth, contrast, colour set, density, text scale applied via data attributes; chart range, default zone, desk order/visibility, bottom bar, landing desk, tooltip delay, hold/stale/offline/trends horizons, band/lights-off/marker toggles, currency, EC + airflow scale wired. Verify on the hotpatched Pi.