refactor(ui): app-wide compact, translucent tooltips

Add material::Tooltip / BeginTooltip / EndTooltip wrappers (tooltip_style.h)
that scope a small window padding (8x4, dpi-scaled) and ~85% opacity to
tooltips only, then route the tooltip call sites through them. Menus and combo
dropdowns are untouched (they keep the global opaque PopupBg).

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

src/ui/windows/mining_stats.cpp

@@ -336,7 +336,7 @@ void RenderMiningStats(const WalletState& state, const MiningInfo& mining,
             if (hov) {
                 dl->AddCircleFilled(btnCenter, btnSize * 0.5f, StateHover());
                 ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
-                ImGui::SetTooltip("%s", toggleTip);
+                material::Tooltip("%s", toggleTip);
             }
             dl->AddText(iconFont, iconFont->LegacySize,
                         ImVec2(btnCenter.x - iconSz.x * 0.5f, btnCenter.y - iconSz.y * 0.5f),