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/import_key_dialog.cpp

@@ -136,7 +136,7 @@ void ImportKeyDialog::render(App* app)
         ImGui::SameLine();
         ImGui::TextDisabled("(?)");
         if (ImGui::IsItemHovered()) {
-            ImGui::SetTooltip("%s", TR("import_key_tooltip"));
+            material::Tooltip("%s", TR("import_key_tooltip"));
         }
 
         // Validation indicator inline with title — check preview text during hover,