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_registry.h

@@ -45,16 +45,30 @@ struct PoolHashrate {
     std::string id;
     double      hashrateHs = 0.0;
     bool        ok = false;
+    // Live pool fee (%) read from the same stats JSON. <0 means "not available" —
+    // callers fall back to the compile-time KnownPool.feePercent.
+    double      feePercent = -1.0;
 };
 
 // The built-in official pools (PPLNS only — never a SOLO pool, whose hashrate is
 // meaningless to balance against). Stable order.
 const std::vector<KnownPool>& knownPools();
 
-// The known pool whose stratum matches `url` (host, and port when both specify one),
-// or nullptr. `url` may be a bare host, host:port, or carry a scheme/userinfo/path.
+// The pool in `pools` whose stratum matches `url` (host, and port when both specify
+// one), or nullptr. `url` may be a bare host, host:port, or carry a scheme/path.
+const KnownPool* findPoolByUrl(const std::vector<KnownPool>& pools, const std::string& url);
+
+// Same, over the built-in official pools only.
 const KnownPool* findKnownPoolByUrl(const std::string& url);
 
+// The full list the UI should show: the official knownPools(), plus a row for every
+// user-saved pool URL and for `currentPoolUrl` when it isn't one of those — so a
+// custom/bookmarked pool is a first-class, selectable row. Synthetic (user) rows are
+// official=false and carry no statsUrl (feePercent<0, no live hashrate), and endpoints
+// are de-duplicated so a saved URL that equals an official pool isn't listed twice.
+std::vector<KnownPool> effectivePools(const std::string& currentPoolUrl,
+                                      const std::vector<std::string>& savedPoolUrls);
+
 // The algo xmrig must use for `url`: the matching known pool's algo, else `fallback`.
 std::string resolvePoolAlgo(const std::string& url, const std::string& fallback);
 
@@ -64,6 +78,13 @@ std::string resolvePoolAlgo(const std::string& url, const std::string& fallback)
 double parsePoolHashrate(PoolStatsSchema schema, const std::string& json,
                          const std::string& miningcorePoolId, bool& ok);
 
+// Parse a pool's advertised fee (%) out of the same stats JSON (DragonXIs:
+// pools.<name>.poolFee; Miningcore: pools[id].poolFeePercent). Selects the same
+// pool entry as parsePoolHashrate. Sets ok=false and returns 0 when the field is
+// absent / malformed, so the caller keeps the compile-time fallback.
+double parsePoolFee(PoolStatsSchema schema, const std::string& json,
+                    const std::string& miningcorePoolId, bool& ok);
+
 // Weighted-random pick among the usable (ok==true) pools: probability is inversely
 // proportional to hashrate (smaller pools favored), so miners spread out instead of
 // all stampeding to the single lowest pool. The current pool (`currentId`, may be