feat(mining): pool registry, schema-aware stats fetcher, pool-select setting

Data + service layer for supporting multiple mining pools:
- pool_registry: KnownPool table (pool.dragonx.is + pool.dragonx.cc) carrying
  each pool's stratum host, xmrig algo, and stats-API schema; pure helpers
  findKnownPoolByUrl / resolvePoolAlgo / parsePoolHashrate / chooseWeightedPool
  (weighted-random by hashrate with incumbent stickiness).
- pool_stats_service: background, schema-aware hashrate fetcher (libcurl, TLS,
  browser UA for Cloudflare) modelled on XmrigUpdater.
- settings: pool_select_mode (manual | auto_balance), string-serialized.
- i18n strings + unit tests (both stats schemas, algo resolution, weighted pick).

Note: pool.dragonx.cc's stratum host is us.dragonx.cc:3333 — the .cc domain is
only the Cloudflare-proxied web/API host and does not accept stratum on :3333.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -500,6 +500,8 @@ set(APP_SOURCES
     src/daemon/xmrig_manager.cpp
     src/util/bootstrap.cpp
     src/util/lite_server_probe.cpp
+    src/util/pool_registry_core.cpp
+    src/util/pool_stats_service.cpp
     src/util/xmrig_updater.cpp
     src/util/xmrig_updater_core.cpp
     src/util/daemon_updater.cpp
@@ -1035,6 +1037,7 @@ if(BUILD_TESTING)
         src/util/platform.cpp
         src/util/logger.cpp
         src/util/lite_server_probe.cpp
+        src/util/pool_registry_core.cpp
         src/util/xmrig_updater.cpp
         src/util/xmrig_updater_core.cpp
         src/util/daemon_updater.cpp