GitLab

Root cause: GetPublicSettingsForInjection used an inline struct that silently
drifted from dto.PublicSettings and omitted channel_monitor_enabled /
available_channels_enabled. On refresh window.__APP_CONFIG__ lacked these
keys, so cachedPublicSettings.available_channels_enabled resolved to
undefined and the opt-in sidebar entry (=== true) disappeared.

Backend: extract PublicSettingsInjectionPayload as a named type with all
feature-flag fields wired, and add a reflect-based drift test in the dto
package so forgetting a future flag fails CI instead of the browser.

Frontend: introduce utils/featureFlags.ts as the single registry for
public-settings-driven toggles, with explicit opt-in / opt-out modes that
encode the pre-load fallback. AppSidebar switches to makeSidebarFlag() so
adding a new switch only touches the registry.