feat(ui): shutdown-screen consistency polish (Phase 3)

The two Phase 3 "bespoke overlays" aren't content modals: the send-error is an
in-tab glass banner (already consistent — left as-is), and the shutdown screen is
a teardown screen where a blur backdrop would be semantically wrong (should read
as "closing") + risky on the teardown path. Per that finding, apply minor
consistency polish to the shutdown screen only:
- Share the overlay-scrim tone via a new material::OverlayScrimColor(opacity),
  used by both the dialog scrim and the shutdown scrim (was a slightly different
  hardcoded 0.06/0.06/0.08 → now the 0.04/0.04/0.06 dialog base).
- "Shutting Down" title → theme Warning() amber (was hardcoded gold).
- Force-Quit confirm destructive button → theme Error() (alpha-varied) instead of
  hardcoded reds.
No behavior change; the shutdown scrim stays opaque (no blur).

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

src/ui/material/draw_helpers.h

@@ -1129,6 +1129,10 @@ inline int SegmentedControl(ImDrawList* dl, ImVec2 origin, float totalW, float h
 inline std::unordered_map<std::string, float> g_overlayCardHeights;
 inline std::string g_overlayCurrentKey;
 
+// The dark base color for any full-window overlay backdrop (dialog scrims + the shutdown screen),
+// so every overlay shares one backdrop tone. `opacity` is the alpha (dialogs vary it per call).
+inline ImVec4 OverlayScrimColor(float opacity) { return ImVec4(0.04f, 0.04f, 0.06f, opacity); }
+
 // Style of an overlay dialog. GlassCard = the classic scrim + glass card + boxed title bar.
 // BlurFloat = the "Manage Portfolio" look: live-blur backdrop + floating content + plain heading.
 enum class OverlayStyle { GlassCard, BlurFloat };
@@ -1173,7 +1177,7 @@ inline bool BeginOverlayDialog(const OverlayDialogSpec& spec)
     if (!overlayPopupOpen) ImGui::SetNextWindowFocus();
     // Blur overlays draw their own backdrop, so the window itself is transparent.
     ImGui::PushStyleColor(ImGuiCol_WindowBg,
-        blur ? ImVec4(0, 0, 0, 0) : ImVec4(0.04f, 0.04f, 0.06f, spec.scrimOpacity));
+        blur ? ImVec4(0, 0, 0, 0) : OverlayScrimColor(spec.scrimOpacity));
     ImGui::PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
     ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0f);
     ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0, 0));