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_pool_panel.cpp

@@ -20,6 +20,17 @@ std::string defaultPoolWorkerAddress(const std::vector<AddressInfo>& addresses)
     return {};
 }
 
+std::string resolveMiningUserAddress(const std::string& payoutAddress,
+                                     const std::string& firstShieldedAddress,
+                                     const std::string& firstTransparentAddress)
+{
+    // The configured payout address is the pool login rewards go to, so it wins over
+    // the wallet's own addresses. "x" is the placeholder for an unset field.
+    if (!payoutAddress.empty() && payoutAddress != "x") return payoutAddress;
+    if (!firstShieldedAddress.empty()) return firstShieldedAddress;
+    return firstTransparentAddress;  // may be empty -> caller reports "no address"
+}
+
 bool miningValueAlreadySaved(const std::vector<std::string>& savedValues,
                              const std::string& value)
 {