Harden a custom chat activity page against CSP eval, polling storms, cache duplication, and fallback error stubs

A browser-based custom chat activity page showed multiple user-visible issues under production conditions: strict CSP blocked a dependency's eval probe, packaged assets referenced missing source maps, polling/subscription refreshes stacked up into repeated slow RPCs, IndexedDB/local/gateway history merges inflated and then collapsed message counts, background/heartbeat sessions could temporarily replace the active chat, transient assistant failure stubs remained visible after successful model fallback, and reload races caused occasional WebSocket handshake warnings.