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>

src/config/settings.h

@@ -60,6 +60,13 @@ public:
         Random
     };
 
+    // Pool selection mode for the mining tab: Manual (user picks the pool) or
+    // AutoBalance (the wallet spreads miners across the official pools by hashrate).
+    enum class PoolSelectMode {
+        Manual,
+        AutoBalance
+    };
+
     struct LiteServerPreference {
         std::string url;
         std::string label;
@@ -305,6 +312,8 @@ public:
     void setPoolHugepages(bool v) { pool_hugepages_ = v; }
     bool getPoolMode() const { return pool_mode_; }
     void setPoolMode(bool v) { pool_mode_ = v; }
+    PoolSelectMode getPoolSelectMode() const { return pool_select_mode_; }
+    void setPoolSelectMode(PoolSelectMode v) { pool_select_mode_ = v; }
 
     // Installed DRG-XMRig release tag (for in-app miner update detection); empty if unknown/bundled.
     std::string getXmrigVersion() const { return xmrig_version_; }
@@ -439,6 +448,7 @@ private:
     bool        pool_tls_       = false;
     bool        pool_hugepages_ = true;
     bool        pool_mode_      = false;   // false=solo, true=pool
+    PoolSelectMode pool_select_mode_ = PoolSelectMode::Manual; // manual vs auto-balance pool choice
     std::string xmrig_version_;            // installed DRG-XMRig release tag (update detection)
     bool        mine_when_idle_ = false;   // auto-start mining when system idle
     int         mine_idle_delay_= 120;     // seconds of idle before mining starts