feat(settings): Bootstrap modal — DialogWarningHeader + centered confirm buttons

Phase 1 (last low-risk modal). Replace the hand-rolled amber warning child-card
with the standard material::DialogWarningHeader (⚠ + Warning color, wrapping),
and center the Download / Mirror / Cancel button row. Already struct-form,
TactileButton, and i18n'd. Completes Phase 1.

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

src/ui/windows/bootstrap_download_dialog.h

@@ -91,21 +91,8 @@ private:
         ImGui::TextWrapped("%s", TR("bootstrap_desc"));
         ImGui::Spacing();
 
-        // Warning card
+        // Care header (reference style: ⚠ + Warning color, wraps if long).
-        ImGui::PushStyleColor(ImGuiCol_ChildBg, ImVec4(0.9f, 0.6f, 0.0f, 0.08f));
+        DialogWarningHeader(TR("bootstrap_warning"));
-        ImGui::BeginChild("##bsWarn", ImVec2(0, 0),
-            ImGuiChildFlags_AutoResizeY | ImGuiChildFlags_AlwaysUseWindowPadding,
-            ImGuiWindowFlags_NoScrollbar);
-        {
-            ImFont* iconFont = Type().iconSmall();
-            ImGui::PushFont(iconFont);
-            ImGui::TextColored(ImGui::ColorConvertU32ToFloat4(Warning()), "%s", ICON_MD_WARNING);
-            ImGui::PopFont();
-            ImGui::SameLine();
-            ImGui::TextWrapped("%s", TR("bootstrap_warning"));
-        }
-        ImGui::EndChild();
-        ImGui::PopStyleColor();
 
         ImGui::Spacing();
 
@@ -124,9 +111,12 @@ private:
         ImGui::Spacing();
         ImGui::Spacing();
 
-        // Buttons: Download | Mirror | Cancel
+        // Buttons: Download | Mirror | Cancel — centered as a group.
         float btnW = 140.0f * dp;
         float btnSm = 90.0f * dp;
+        const float rowTotal = btnW * 2.0f + btnSm + ImGui::GetStyle().ItemSpacing.x * 2.0f;
+        const float rowOff = (ImGui::GetContentRegionAvail().x - rowTotal) * 0.5f;
+        if (rowOff > 0.0f) ImGui::SetCursorPosX(ImGui::GetCursorPosX() + rowOff);
 
         if (TactileButton(TR("download"), ImVec2(btnW, 0))) {
             startDownload("");