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_tab_helpers.h

@@ -9,6 +9,7 @@ int GetMaxMiningThreads();
 int ClampMiningThreads(int requestedThreads, int maxThreads);
 bool IsPoolMiningActive(bool poolMode, bool xmrigRunning, bool soloMiningRunning);
 std::string FormatHashrate(double hashrate);
+std::string FormatFeePercent(double feePercent);
 double EstimateHoursToBlock(double localHashrate, double networkHashrate, double difficulty);
 std::string FormatEstTime(double estimatedHours);