Improve auth recovery UX after sync failures

[davedean]

Motivation

• Background sync failures that are authentication-related currently leave the UI in a confusing state with lists still visible and the user forced to manually sign out and back in.
• The app should surface auth failures immediately and guide the user to re-authenticate so background-triggered token invalidation doesn't cause stale UI or a hidden sign-in flow.

Description

• Make the main UI require sign-in by presenting a blocking LoginView when there is no valid token and hide the Lists/Sync sections while signed out.
• Add an allowsDismiss flag to LoginView so it can be used as a dismissible sheet or a non-dismissible full-screen cover for blocking sign-in.
• Detect Vikunja auth errors (401/403) via ErrorPresenter.isAuthenticationError(_:) and clear the token + surface the login flow when those occur during manual sync, project loading, or background sync.
• Persist an auth-specific flag by extending BackgroundSyncStatus with a backward-compatible requiresReauth field and delete the keychain token from background sync when an auth failure is detected.
• Add reloadTokenFromKeychain() to AppState and call it on app foreground to reflect background invalidation in the UI, and clear cachedProjects when the token is removed.

Testing

• Ran the package test suite with swift test, which built successfully and all unit tests passed (70 tests, 0 failures).
• Verified the app compiles and the changes were committed (manual smoke verification steps listed in the PR body during commit creation were followed where possible in this environment).

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Codex Task