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/receive_tab.cpp

@@ -274,7 +274,7 @@ static void RenderAddressDropdown(App* app, float width) {
                         s_cached_qr_data.clear(); // Force QR regeneration
                     }
                     if (ImGui::IsItemHovered()) {
-                        ImGui::SetTooltip("%s\nBalance: %.8f %s%s",
+                        material::Tooltip("%s\nBalance: %.8f %s%s",
                             addr.address.c_str(), addr.balance, DRAGONX_TICKER,
                             isCurrent ? "\n(selected)" : "");
                     }
@@ -842,7 +842,7 @@ void RenderReceiveTab(App* app)
                 ImGui::InvisibleButton("##QRClickCopy", ImVec2(totalQrSize, totalQrSize));
                 if (ImGui::IsItemHovered()) {
                     ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
-                    ImGui::SetTooltip("%s",
+                    material::Tooltip("%s",
                         s_request_amount > 0 ? TR("click_copy_uri") : TR("click_copy_address"));
                 }
                 if (ImGui::IsItemClicked()) {