Northline RP Community Portal v2.9.7

Maintenance gate hardening

• Added a client-side maintenance access guard that re-checks access after client-side navigation.
• Added /api/maintenance/access so already-open Tweeter sessions cannot navigate into the main site while maintenance mode is active.
• Converted Tweeter's direct "Back to Northline" links to normal full-page anchors so the server-side maintenance page is enforced immediately.
• Keeps the existing behavior where Tweeter can remain available during main-site maintenance when that setting is enabled.

This fixes the case where a user could open Tweeter while it was allowed through maintenance mode, then use client-side navigation to view other Northline pages until refreshing.