refactor(settings): import dialog layout polish

Several layout refinements to the Import Private/Viewing Key dialog:
- Widen the dialog to 640px so the one-line warning under the heading no longer wraps.
- Vertically center DialogWarningHeader's label with the (taller) warning icon
  instead of top-aligning it — a consistent fix for every warning header.
- Add a small gap between the key input's text and the flush eye toggle.
- Center + slightly enlarge the Paste / Clear buttons.
- Center the Import/Sweep + Close action buttons.
- Scan-height control: put the numeric input first with the slider below it, and
  drop the redundant "0 = rescan from the start" hint (0 is the field default; the
  transparent-key note is kept).

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

src/ui/material/draw_helpers.h

@@ -1666,10 +1666,16 @@ inline void BeginOverlayDialogFooter(float totalActionWidth, bool drawSeparator
 // TextColored(label).  The label text is passed in (already translated).
 inline void DialogWarningHeader(const char* warningLabel, const ImVec4& col = WarningVec4())
 {
-    ImGui::PushFont(Type().iconLarge());
+    ImFont* iconF = Type().iconLarge();
+    const float rowTop = ImGui::GetCursorPosY();
+    const float iconH  = iconF->LegacySize;      // already dp-scaled
+    const float textH  = ImGui::GetFontSize();
+    ImGui::PushFont(iconF);
     ImGui::TextColored(col, ICON_MD_WARNING);
     ImGui::PopFont();
     ImGui::SameLine();
+    // Vertically center the label with the taller warning glyph (stays within the icon's row height).
+    if (iconH > textH) ImGui::SetCursorPosY(rowTop + (iconH - textH) * 0.5f);
     ImGui::TextColored(col, "%s", warningLabel);
 }