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/transactions_tab.cpp

@@ -842,7 +842,7 @@ void RenderTransactionsTab(App* app)
 
                     // Tooltip
                     if (ImGui::IsItemHovered()) {
-                        ImGui::SetTooltip("%s\n%s\n%s", tx.address.c_str(),
+                        material::Tooltip("%s\n%s\n%s", tx.address.c_str(),
                             tx.txid.c_str(), tx.getTimeString().c_str());
                     }