feat: unify UI preference persistence (DB source of truth)

[clFaster]

Summary #23

• add a shared useUiPreferences hook to centralize UI preference state and persistence
• switch sidebar collapsed, hide export row, and show timestamps preferences to DB-backed settings
• remove duplicated localStorage writes for preferences now persisted in DB
• add migration fallback from legacy localStorage values when DB values are missing
• extend system e2e coverage with sidebar collapse persistence across app restart

Verification

• pnpm format
• pnpm lint
• pnpm test:system

Notes

• CodeScene MCP analysis could not be executed in this environment because no access token is configured.