feat(material): promote blur-overlay modal primitives (Phase 0a)

First step toward unifying all app modals on the Manage-Portfolio visual style.
Add, as shared material:: helpers (additive — no caller yet, no visible change):
- blur-overlay open refcount + 1-frame latch (Push/Pop/AnyBlurOverlayActive[LastFrame],
  LatchBlurOverlayActive) so App::render's effect guard can be modal-agnostic;
- capture-frame query (IsCapturingBlurBackdrop) for scroll-fade suppression;
- RightAlignX and SegmentedControl (promoted verbatim from market_tab);
- BeginFadeScrollChild/EndFadeScrollChild in a dedicated overlay_scroll.h (kept
  out of the widely-included draw_helpers.h because effects::ScrollFadeShader's GL
  headers clash with SDL's in some TUs).
Also fix IsCurrentWindowOverlayDialog to prefix-match "##OverlayScrim" so suffixed
overlay windows are recognized by the modal-aware hover check.

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

src/ui/material/overlay_scroll.h

@@ -0,0 +1,68 @@
+// DragonX Wallet - ImGui Edition
+// Copyright 2024-2026 The Hush Developers
+// Released under the GPLv3
+
+#pragma once
+
+// Edge-fade scroll region for the blur-backdrop modals (smooth scroll + Settings-tab edge fade +
+// a thicker, inset, rounded scrollbar). Split out of draw_helpers.h because it depends on
+// effects::ScrollFadeShader, whose GL/DX headers must not be pulled into the widely-included base
+// header (they clash with SDL's GL headers in some TUs). Include this only from modal .cpp files.
+
+#include "draw_helpers.h"                    // IsCapturingBlurBackdrop, ApplySmoothScroll
+#include "../effects/scroll_fade_shader.h"
+#include "../effects/low_spec.h"
+#include "imgui.h"
+#include <algorithm>
+#include <string>
+
+namespace dragonx {
+namespace ui {
+namespace material {
+
+// Scroll region with smooth scrolling + edge fade + a thicker, inset, rounded scrollbar. Uses a
+// bordered/rounded OUTER frame whose padding insets the scrollbar, holding an INNER scrolling child.
+// `fade` must persist across frames (one instance per scroll region); `pad` is the inner content
+// inset. Returns the inner draw list; pair with EndFadeScrollChild().
+inline ImDrawList* BeginFadeScrollChild(const char* id, effects::ScrollFadeShader& fade,
+                                        const ImVec2& pad, float dp)
+{
+    ImGui::PushStyleVar(ImGuiStyleVar_ChildRounding, 6.0f * dp);
+    ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(6.0f * dp, 6.0f * dp));
+    ImGui::BeginChild(id, ImVec2(0, std::max(120.0f * dp, ImGui::GetContentRegionAvail().y)), true,
+                      ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoScrollWithMouse);
+    ImGui::PopStyleVar(2);  // ChildRounding + outer WindowPadding (captured by the outer child)
+    ImGui::PushStyleVar(ImGuiStyleVar_ScrollbarSize, 11.0f * dp);
+    ImGui::PushStyleVar(ImGuiStyleVar_ScrollbarRounding, 5.5f * dp);
+    ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, pad);
+    std::string innerId = std::string(id) + "_i";
+    ImGui::BeginChild(innerId.c_str(), ImVec2(0, 0), false, ImGuiWindowFlags_NoScrollWithMouse);
+    ImGui::PopStyleVar();   // inner WindowPadding — captured by this child; don't cascade to tooltips
+    ApplySmoothScroll();
+    ImDrawList* dl = ImGui::GetWindowDrawList();
+    const float fadeH = 22.0f * dp;
+    if (!effects::isLowSpecMode() && !IsCapturingBlurBackdrop() && fade.init()) {
+        ImVec2 cMin = ImGui::GetWindowPos();
+        ImVec2 cMax(cMin.x + ImGui::GetWindowSize().x, cMin.y + ImGui::GetWindowSize().y);
+        float sY = ImGui::GetScrollY(), sMax = ImGui::GetScrollMaxY();
+        fade.fadeTopY       = cMin.y;
+        fade.fadeBottomY    = cMax.y;
+        fade.fadeZoneTop    = (sY > 1.0f) ? fadeH : 0.0f;
+        fade.fadeZoneBottom = (sMax > 0.0f && sY < sMax - 1.0f) ? fadeH : 0.0f;
+        fade.addBind(dl);
+    }
+    return dl;
+}
+
+inline void EndFadeScrollChild(effects::ScrollFadeShader& fade, ImDrawList* dl)
+{
+    if (!effects::isLowSpecMode() && !IsCapturingBlurBackdrop() && fade.ready)
+        effects::ScrollFadeShader::addUnbind(dl);
+    ImGui::EndChild();       // inner
+    ImGui::PopStyleVar(2);   // ScrollbarSize + ScrollbarRounding
+    ImGui::EndChild();       // outer
+}
+
+} // namespace material
+} // namespace ui
+} // namespace dragonx