refactor(ui): add material::IconButton helper + adopt at 8 frameless icon buttons

UI-standardization audit (icon-button discovery pass): the app had ~22 clean
hand-rolled icon buttons repeating the same "InvisibleButton hit-target +
centered glyph + hover feedback + tooltip" boilerplate. Add a shared
material::IconButton(id, glyph, font, size, IconButtonStyle) — centered glyph,
optional resting + hover backgrounds (pill/rounded-rect), hover recolor, hand
cursor + tooltip; caller resolves state-dependent glyph/colors and returns
clicked. Adopt at the homogeneous, self-contained sites, pixel-faithful:
- mining_mode_toggle: pool/worker dropdown + bookmark + reset (×5)
- network: server hide/unhide eye (×1)
- mining_controls: pool benchmark start (×1)
- mining_stats: pool-balance refresh (×1)

Deferred (own follow-up + sweep): the idle/scale/gpu active-pill toggles use a
"draw pill+glyph earlier, hit-test later" idiom needing restructuring; the
raw-rect (IsRectHovered, no InvisibleButton) sites in market/mining_stats/
balance are a different hit-test mechanism left intentionally. Framed icon
pagers already ride TactileButton after the prior commit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/mining_stats.cpp

@@ -258,24 +258,16 @@ static void RenderLeftPoolCard(App* app, const WalletState& state, ImDrawList* d
                  (int)util::knownPools().size());
         dl->AddText(ovFont, ovFont->LegacySize, ImVec2(x, y), OnSurfaceMedium(), hdr);
 
-        ImFont* icoFont = Type().iconSmall();
-        const char* icon = ICON_MD_REFRESH;
-        ImVec2 iSz = icoFont->CalcTextSizeA(icoFont->LegacySize, FLT_MAX, 0, icon);
         float btnS = ovFont->LegacySize + 6 * dp;
         ImVec2 rbMin(x + w - btnS, y - 2 * dp);
-        ImVec2 rbMax(x + w, y - 2 * dp + btnS);
+        material::IconButtonStyle st;
+        st.color = OnSurfaceMedium();
+        st.hoverBg = StateHover();
+        st.bgRounding = 3 * dp;
+        st.tooltip = TR("mining_refresh");
         ImGui::SetCursorScreenPos(rbMin);
-        ImGui::InvisibleButton("##PoolRefresh", ImVec2(btnS, btnS));
-        bool rhov = ImGui::IsItemHovered();
-        if (rhov) {
-            dl->AddRectFilled(rbMin, rbMax, StateHover(), 3 * dp);
-            ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
-            material::Tooltip("%s", TR("mining_refresh"));
-        }
-        dl->AddText(icoFont, icoFont->LegacySize,
-                    ImVec2((rbMin.x + rbMax.x - iSz.x) * 0.5f, (rbMin.y + rbMax.y - iSz.y) * 0.5f),
-                    OnSurfaceMedium(), icon);
-        if (ImGui::IsItemClicked()) app->requestPoolBalanceRefresh();
+        if (material::IconButton("##PoolRefresh", ICON_MD_REFRESH, Type().iconSmall(), ImVec2(btnS, btnS), st))
+            app->requestPoolBalanceRefresh();
     }
     y += ovFont->LegacySize + 4 * dp;