refactor: remove object environment dependencies

[everpcpc]

Background

• KMReader still had non-optional object-style environment lookups across app shells, navigation roots, sheets/covers, and the macOS reader window path.
• In this codebase those dependencies are fragile during snapshot, rotation, scene transitions, and hosting boundaries, which matches the SwiftUI environment-missing crash family we were chasing on iPad.
• Auth and reader state were also spread across multiple injection paths and duplicated reader window state, which made the control flow harder to reason about and easier to break.

Summary

• remove non-optional object-style environment dependencies from the app shell and navigation roots, and pass auth and reader dependencies explicitly through AppViewContext, tab roots, split roots, and onboarding flows
• keep auth session facts in Current and AppStorage, keep AuthViewModel focused on bootstrap and auth flows, and make root readiness depend on explicit bootstrap state instead of implicit environment wiring
• replace ReaderWindowManager and BookReaderState with a single ReaderSession source inside ReaderPresentationManager, so iOS/tvOS overlays and the macOS reader window all read the same session state
• introduce ReaderActions for leaf reader entry points so book cards, book actions, and series actions depend on explicit read actions instead of the whole presentation coordinator
• tighten AGENTS.md guidance to ban non-optional object-style environment dependencies in app code

Testing

• make format
• make build