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_controls.cpp

@@ -487,27 +487,12 @@ void RenderMiningControls(App* app, const WalletState& state, const MiningInfo&
                     float btnX  = idleRightEdge - btnSz;
                     float btnY  = curY + (headerH - btnSz) * 0.5f;
 
+                    material::IconButtonStyle st;
+                    st.color = OnSurfaceMedium();
+                    st.hoverBg = StateHover();
+                    st.tooltip = TR("mining_benchmark_tooltip");
                     ImGui::SetCursorScreenPos(ImVec2(btnX, btnY));
-                    ImGui::InvisibleButton("##BenchStart", ImVec2(btnSz, btnSz));
-                    bool benchHovered = ImGui::IsItemHovered();
-                    bool benchClicked = ImGui::IsItemClicked();
-
-                    // Hover highlight
-                    if (benchHovered) {
-                        dl->AddRectFilled(ImVec2(btnX, btnY), ImVec2(btnX + btnSz, btnY + btnSz),
-                                          StateHover(), btnSz * 0.5f);
-                        ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
-                        material::Tooltip("%s", TR("mining_benchmark_tooltip"));
-                    }
-
-                    const char* benchIcon = ICON_MD_SPEED;
-                    ImVec2 bIcoSz = icoFont->CalcTextSizeA(icoFont->LegacySize, FLT_MAX, 0, benchIcon);
-                    dl->AddText(icoFont, icoFont->LegacySize,
-                                ImVec2(btnX + (btnSz - bIcoSz.x) * 0.5f,
-                                       btnY + (btnSz - bIcoSz.y) * 0.5f),
-                                OnSurfaceMedium(), benchIcon);
-
-                    if (benchClicked) {
+                    if (material::IconButton("##BenchStart", ICON_MD_SPEED, icoFont, ImVec2(btnSz, btnSz), st)) {
                         // Require a wallet address for pool mining
                         std::string worker(s_pool_worker);
                         if (!worker.empty()) {