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/ui/windows/mining_stats.cpp

@@ -251,11 +251,15 @@ static void RenderLeftPoolCard(App* app, const WalletState& state, ImDrawList* d
     }
     y += gap * 0.5f;
 
+    // The pool list = official pools ∪ user-saved favorites ∪ the current custom pool.
+    const auto effective = util::effectivePools(app->settings()->getPoolUrl(),
+                                                app->settings()->getSavedPoolUrls());
+
     // --- POOLS (N) header + Refresh ---
     {
         char hdr[48];
         snprintf(hdr, sizeof(hdr), "%s (%d)", TR("mining_pools_header"),
-                 (int)util::knownPools().size());
+                 (int)effective.size());
         dl->AddText(ovFont, ovFont->LegacySize, ImVec2(x, y), OnSurfaceMedium(), hdr);
 
         float btnS = ovFont->LegacySize + 6 * dp;
@@ -278,11 +282,11 @@ static void RenderLeftPoolCard(App* app, const WalletState& state, ImDrawList* d
     {
         ImDrawList* cdl = ImGui::GetWindowDrawList();
         const auto snap = app->poolStatsSnapshot();
-        const util::KnownPool* current = util::findKnownPoolByUrl(app->settings()->getPoolUrl());
+        const util::KnownPool* current = util::findPoolByUrl(effective, app->settings()->getPoolUrl());
         const float childW = ImGui::GetContentRegionAvail().x;
         const float listRowH = capFont->LegacySize + 10 * dp;
 
-        for (const auto& kp : util::knownPools()) {
+        for (const auto& kp : effective) {
             ImGui::PushID(kp.id.c_str());
             const bool isCurrent = current && current->id == kp.id;
             const auto it = snap.byId.find(kp.id);
@@ -315,7 +319,17 @@ static void RenderLeftPoolCard(App* app, const WalletState& state, ImDrawList* d
 
             char right[64];
             std::string hrStr = haveHr ? FormatHashrate(it->second.hashrateHs) : std::string("—");
-            snprintf(right, sizeof(right), "%s  %.0f%% fee", hrStr.c_str(), kp.feePercent);
+            // Prefer the live fee the pool reports; fall back to the compile-time
+            // KnownPool.feePercent. A synthetic user pool has an unknown (<0) fee, so
+            // we show just its hashrate placeholder for it.
+            double feePct = (it != snap.byId.end() && it->second.feePercent >= 0.0)
+                                ? it->second.feePercent
+                                : kp.feePercent;
+            if (feePct >= 0.0)
+                snprintf(right, sizeof(right), "%s  %s%% fee", hrStr.c_str(),
+                         FormatFeePercent(feePct).c_str());
+            else
+                snprintf(right, sizeof(right), "%s", hrStr.c_str());
             ImVec2 rSz = capFont->CalcTextSizeA(capFont->LegacySize, FLT_MAX, 0, right);
             cdl->AddText(capFont, capFont->LegacySize,
                          ImVec2(rMax.x - rSz.x - 6 * dp, textY), OnSurfaceMedium(), right);