fix(ui): overview/mining polish + default-pool cleanup

- balance: inset the address-row favorite (star) button by the card's inner
  padding so it mirrors the left margin instead of hugging the card edge.
- mining: remove pool.dragonx.cc from the built-in default pools (pool.dragonx.is
  is now the sole default); update the pool-registry test accordingly.
- mining: middle-truncate saved pool-URL and payout-address dropdown rows (new
  shared material::TruncateToWidth helper) so a full z-address no longer runs
  under the trailing delete (X) button.
- mining: fix the thread-grid cells overflowing the card at >100% display
  scaling — the reserved Mine-button width used a raw clamp that didn't scale;
  scale it by dp so cols is estimated correctly (no-op at 100%).

Full-node build + test suite green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ui/material/draw_helpers.h

@@ -57,6 +57,21 @@ inline ImU32 ReadableError() {
     return IM_COL32(r, g, b, (e >> IM_COL32_A_SHIFT) & 0xFF);
 }
 
+// Middle-ellipsis truncation ("front...back", roughly equal halves) so `text` fits within
+// maxWidth pixels when drawn with `font` at `fontSize`. Returns `text` unchanged if it already
+// fits (or maxWidth is non-positive). Display-only — never mutate the underlying value with this.
+inline std::string TruncateToWidth(const std::string& text, ImFont* font, float fontSize, float maxWidth) {
+    if (text.empty() || !font || maxWidth <= 0.0f) return text;
+    if (font->CalcTextSizeA(fontSize, FLT_MAX, 0.0f, text.c_str()).x <= maxWidth) return text;
+    const int n = static_cast<int>(text.size());
+    for (int f = n / 2; f >= 3; --f) {
+        const int b = (f - 2 > 3) ? (f - 2) : 3;             // keep the two halves roughly equal
+        std::string t = text.substr(0, f) + "..." + text.substr(n - b);
+        if (font->CalcTextSizeA(fontSize, FLT_MAX, 0.0f, t.c_str()).x <= maxWidth) return t;
+    }
+    return n > 6 ? (text.substr(0, 3) + "..." + text.substr(n - 3)) : text;
+}
+
 // Animated "loading" ellipsis: "", ".", "..", "..." cycling on a ~3Hz phase.
 inline const char* LoadingDots() {
     int n = ((int)(ImGui::GetTime() * 3.0f)) % 4;