Remove legacy localStorage migration from UI preferences hook

[clFaster]

Goal

Remove temporary localStorage migration fallback from UI preferences once rollout is complete

Type

Technical

Effort

S

Impact

Low

Evidence

useUiPreferences still contains legacy localStorage migration helpers (LEGACY_STORAGE_KEYS, readLegacyPreferences, migrateLegacyPreferences) in src/hooks/useUiPreferences.ts to preserve settings from pre-DB persistence.

First Implementation Step

After one stable release cycle with DB-backed preferences, remove legacy read/migrate code and keep DB defaults as the only fallback path.