feat(mining): live pool fee, saved/custom pool rows, and payout-address fix

Several related mining-tab pool improvements:

- Report the default pool fee correctly: pool.dragonx.is is 1%, not 0%.
  The registry constant was hardcoded to 0. It now also fetches the live
  poolFee from the pool's /api/stats alongside hashrate (no extra
  request), so the displayed fee self-corrects and falls back to the
  compile-time value only when the fetch hasn't landed.

- Show fractional fees: new FormatFeePercent trims trailing zeros so
  whole fees read "1%" and fractional ones keep their decimals ("1.5%").

- Surface saved + custom pools in the pool list card: the list is now
  the union of the official pools, the user's saved favorites, and the
  currently-mined pool (effectivePools), each a selectable, endpoint-
  deduped row. Previously the card only showed the hardcoded knownPools().

- Fix the xmrig "user" field: the "Payout Address" field now drives the
  pool login rewards are credited to (resolveMiningUserAddress), instead
  of being written only to "pass" while "user" was auto-derived from the
  wallet's own first z-address -- which silently ignored a configured
  payout address and could route rewards to the wrong address.

Unit tests cover parsePoolFee, FormatFeePercent, effectivePools, and
resolveMiningUserAddress; full app + ObsidianDragonTests build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/pool_registry_core.cpp

@@ -28,7 +28,7 @@ const std::vector<KnownPool>& knownPools()
         KnownPool{
             "dragonx-is", "pool.dragonx.is", "pool.dragonx.is:3433", "rx/hush",
             "https://pool.dragonx.is/api/stats", PoolStatsSchema::DragonXIs,
-            /*miningcorePoolId=*/"", /*feePercent=*/0.0, /*official=*/true,
+            /*miningcorePoolId=*/"", /*feePercent=*/1.0, /*official=*/true,
         },
     };
     return pools;
@@ -83,13 +83,62 @@ bool sameEndpoint(const std::string& a, const std::string& b)
     return pa == pb;
 }
 
+// Build a synthetic, selectable pool row for a user-supplied URL (a saved favorite
+// or the current custom pool). We don't know its stats API, so it carries no
+// statsUrl / live hashrate and an unknown (<0) fee — the UI falls back to "—".
+KnownPool makeUserPool(const std::string& url)
+{
+    KnownPool p;
+    const std::string hp = hostPortOf(url);
+    std::string host, port;
+    splitHostPort(hp, host, port);
+    p.id       = "user:" + trimmed(url);  // stable + unique (used as the ImGui id)
+    p.label    = host.empty() ? hp : host;
+    p.stratum  = trimmed(url);            // what the miner connects to / a row-click restores
+    p.algo     = "";                      // unknown; xmrig resolves via resolvePoolAlgo's fallback
+    p.statsUrl = "";                      // no known stats endpoint -> no live hashrate/fee
+    p.schema   = PoolStatsSchema::DragonXIs;
+    p.miningcorePoolId = "";
+    p.feePercent = -1.0;                  // unknown fee
+    p.official = false;
+    return p;
+}
+
 } // namespace
 
+const KnownPool* findPoolByUrl(const std::vector<KnownPool>& pools, const std::string& url)
+{
+    for (const auto& p : pools)
+        if (sameEndpoint(p.stratum, url)) return &p;
+    return nullptr;
+}
+
 const KnownPool* findKnownPoolByUrl(const std::string& url)
 {
-    for (const auto& p : knownPools())
-        if (sameEndpoint(p.stratum, url)) return &p;
-    return nullptr;
+    return findPoolByUrl(knownPools(), url);
+}
+
+std::vector<KnownPool> effectivePools(const std::string& currentPoolUrl,
+                                      const std::vector<std::string>& savedPoolUrls)
+{
+    std::vector<KnownPool> pools = knownPools();
+
+    // Skip anything whose endpoint already appears (official or an earlier user row).
+    auto listed = [&](const std::string& url) {
+        return findPoolByUrl(pools, url) != nullptr;
+    };
+
+    for (const auto& url : savedPoolUrls) {
+        if (trimmed(url).empty() || listed(url)) continue;
+        pools.push_back(makeUserPool(url));
+    }
+
+    // The pool currently being mined, if not already shown, so the active pool is
+    // always visible even before it's bookmarked.
+    if (!trimmed(currentPoolUrl).empty() && !listed(currentPoolUrl))
+        pools.push_back(makeUserPool(currentPoolUrl));
+
+    return pools;
 }
 
 std::string resolvePoolAlgo(const std::string& url, const std::string& fallback)
@@ -159,6 +208,64 @@ double parsePoolHashrate(PoolStatsSchema schema, const std::string& jsonStr,
     return 0.0;
 }
 
+double parsePoolFee(PoolStatsSchema schema, const std::string& jsonStr,
+                    const std::string& miningcorePoolId, bool& ok)
+{
+    ok = false;
+    try {
+        const json j = json::parse(jsonStr);
+
+        if (schema == PoolStatsSchema::DragonXIs) {
+            // { "pools": { "dragonx": { "poolFee": <num>, ... }, ... } }
+            if (j.contains("pools") && j["pools"].is_object()) {
+                const auto& pools = j["pools"];
+                auto readFee = [&](const json& pool, double& out) -> bool {
+                    if (pool.is_object() && pool.contains("poolFee") &&
+                        pool["poolFee"].is_number()) {
+                        out = pool["poolFee"].get<double>();
+                        return true;
+                    }
+                    return false;
+                };
+                double fee = 0.0;
+                if (pools.contains("dragonx") && readFee(pools["dragonx"], fee)) {
+                    ok = true;
+                    return fee;
+                }
+                for (auto it = pools.begin(); it != pools.end(); ++it) {
+                    if (readFee(it.value(), fee)) {
+                        ok = true;
+                        return fee;
+                    }
+                }
+            }
+        } else {  // Miningcore: pools[id].poolFeePercent
+            if (j.contains("pools") && j["pools"].is_array()) {
+                const json* chosen = nullptr;
+                for (const auto& pool : j["pools"]) {
+                    if (!pool.is_object()) continue;
+                    if (!miningcorePoolId.empty()) {
+                        if (pool.value("id", std::string{}) == miningcorePoolId) {
+                            chosen = &pool;
+                            break;
+                        }
+                    } else if (!chosen) {
+                        chosen = &pool;  // first pool when no id requested
+                    }
+                }
+                if (chosen && chosen->contains("poolFeePercent") &&
+                    (*chosen)["poolFeePercent"].is_number()) {
+                    ok = true;
+                    return (*chosen)["poolFeePercent"].get<double>();
+                }
+            }
+        }
+    } catch (...) {
+        // fall through — ok stays false
+    }
+    return 0.0;
+}
+
 std::string chooseWeightedPool(const std::vector<PoolHashrate>& pools,
                                const std::string& currentId,
                                std::mt19937& rng)