Fix network requests hanging after app is backgrounded and reopened

[mzaki9]

After backgrounding the app and reopening it, network requests sometimes hang forever (manga pages keep spinning) until the app is force-stopped. This is the known OkHttp stale-connection issue: idle keep-alive connections in the connection pool die while the app is backgrounded (WiFi power-save, radio recycling, network switches) and requests routed to them hang silently until the 5-minute idle reap. With DNS-over-HTTPS enabled, the DoH resolver client builds its own client via build(), so it has a second, separate pool that nothing ever evicts.

Changes:

• Share a single ConnectionPool between the main client and the DoH resolver client (set on the builder before the DoH provider block), so one evictAll() clears both pools.
• Evict the pool on network transitions (internet lost/regained) via a NetworkCallback registered with registerDefaultNetworkCallback.
• Evict the pool when the app returns to the foreground (ProcessLifecycleOwner ON_START).

Testing:

1. Open a manga, background the app, toggle WiFi or let the device sleep, reopen.
2. Manga pages load without force-stopping the app.