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.cpp

@@ -41,6 +41,21 @@ std::string FormatHashrate(double hashrate)
     return std::string(buffer);
 }
 
+std::string FormatFeePercent(double feePercent)
+{
+    // Whole fees read "1"; fractional ones keep only their significant decimals
+    // ("1.5", "0.9", "1.25") with no trailing zeros. Capped at 2 dp — finer than
+    // any pool advertises, and the caller appends the "%".
+    char buffer[32];
+    snprintf(buffer, sizeof(buffer), "%.2f", feePercent);
+    std::string s(buffer);
+    if (s.find('.') != std::string::npos) {
+        s.erase(s.find_last_not_of('0') + 1);
+        if (!s.empty() && s.back() == '.') s.pop_back();
+    }
+    return s;
+}
+
 double EstimateHoursToBlock(double localHashrate, double networkHashrate, double difficulty)
 {
     (void)difficulty;