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/material/draw_helpers.h

@@ -354,6 +354,47 @@ inline ImVec2 DrawPill(ImDrawList* dl, const ImVec2& pos, const char* text, ImFo
     return sz;
 }
 
+// ── Frameless icon button ────────────────────────────────────────────────
+// Style for IconButton. All colors default to 0 (= unset): color falls back to OnSurfaceMedium();
+// hoverColor==0 means "no hover recolor"; hoverBg/restBg==0 mean "no background".
+struct IconButtonStyle {
+    ImU32 color = 0;          // resting glyph color (0 => OnSurfaceMedium())
+    ImU32 hoverColor = 0;     // glyph color while hovered (0 => same as color)
+    ImU32 hoverBg = 0;        // fill drawn behind the glyph on hover only (0 => none)
+    ImU32 restBg = 0;         // fill drawn every frame, e.g. an "active" pill (0 => none)
+    float bgRounding = -1.0f; // <0 => size.y*0.5 (pill/circle for a square); else that radius
+    bool  handCursor = true;
+    const char* tooltip = nullptr;
+};
+
+// A frameless / pill icon button: an InvisibleButton(id, size) at the current cursor with a centered
+// `glyph` (in `font`), optional resting + hover backgrounds, hover recolor, hand cursor + tooltip.
+// Returns true the frame it is clicked. The caller resolves any state-dependent glyph/colors/tooltip
+// before the call and positions the cursor (SetCursorScreenPos / SameLine) as needed.
+inline bool IconButton(const char* id, const char* glyph, ImFont* font, const ImVec2& size,
+                       const IconButtonStyle& st)
+{
+    const ImVec2 pos = ImGui::GetCursorScreenPos();
+    ImGui::InvisibleButton(id, size);
+    const bool hov = ImGui::IsItemHovered();
+    const bool clk = ImGui::IsItemClicked();
+    ImDrawList* dl = ImGui::GetWindowDrawList();
+    const ImVec2 pMax(pos.x + size.x, pos.y + size.y);
+    const float round = st.bgRounding < 0.0f ? size.y * 0.5f : st.bgRounding;
+    if (st.restBg != 0)         dl->AddRectFilled(pos, pMax, st.restBg, round);
+    if (hov && st.hoverBg != 0) dl->AddRectFilled(pos, pMax, st.hoverBg, round);
+    if (hov) {
+        if (st.handCursor) ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
+        if (st.tooltip && st.tooltip[0]) Tooltip("%s", st.tooltip);
+    }
+    const ImU32 gcol = (hov && st.hoverColor != 0) ? st.hoverColor
+                     : (st.color != 0 ? st.color : OnSurfaceMedium());
+    const ImVec2 gsz = font->CalcTextSizeA(font->LegacySize, FLT_MAX, 0, glyph);
+    dl->AddText(font, font->LegacySize,
+                ImVec2(pos.x + (size.x - gsz.x) * 0.5f, pos.y + (size.y - gsz.y) * 0.5f), gcol, glyph);
+    return clk;
+}
+
 // ── Tactile wrappers ────────────────────────────────────────────────────
 // Drop-in replacements for ImGui::Button / SmallButton that automatically
 // add the tactile highlight overlay after rendering.