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_stats_service.cpp

@@ -94,6 +94,12 @@ void PoolStatsService::run(std::vector<KnownPool> pools)
             const double v = parsePoolHashrate(p.schema, body, p.miningcorePoolId, ok);
             hr.ok = ok;
             hr.hashrateHs = ok ? v : 0.0;
+
+            bool feeOk = false;
+            const double fee = parsePoolFee(p.schema, body, p.miningcorePoolId, feeOk);
+            // Only trust a sane fee; anything else leaves feePercent < 0 so the UI
+            // falls back to the compile-time KnownPool.feePercent.
+            if (feeOk && fee >= 0.0 && fee <= 100.0) hr.feePercent = fee;
         }
         results[p.id] = hr;
     }