fix(modals): vertically center auto-height dialogs in the window

Fixed-height overlay cards were already centered, but auto-height cards
hard-anchored at 15% from the top, so shorter dialogs (e.g. Migrate to a seed
wallet, Import Key, About) rendered top-heavy with a large empty area below.

Center auto-height cards on last frame's measured content height (already cached
per dialog id in g_overlayCardHeights). A re-opened dialog centers from frame 1;
only the very first open of a given dialog this session briefly anchors near the
top before it re-centers. Content taller than the window top-anchors at a 16px
margin. One framework change centers every modal (verified across migrate /
import-key / about / seed-backup at 100%).

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

src/ui/material/draw_helpers.h

@@ -1396,8 +1396,10 @@ inline bool BeginOverlayDialog(const OverlayDialogSpec& spec)
                            ImGuiButtonFlags_MouseButtonRight |
                            ImGuiButtonFlags_MouseButtonMiddle);
 
-    // Card geometry: fixed-height (large modals) is centered; auto-height reuses last frame's
+    // Card geometry: both fixed- and auto-height cards are VERTICALLY CENTERED in the window.
-    // measured content height (short dialogs get short cards), capped at the viewport ratio.
+    // Fixed-height centers its known height; auto-height centers last frame's measured content
+    // height (cached per dialog id, so a re-opened dialog is centered from frame 1 — only the very
+    // first open of a given dialog this session briefly anchors near the top before it re-centers).
     float cardX = vp_pos.x + (vp_size.x - cardWidth) * 0.5f;
     float cardY, cardBottomY;
     const bool fixedHeight = (spec.cardHeight > 0.0f);
@@ -1407,12 +1409,20 @@ inline bool BeginOverlayDialog(const OverlayDialogSpec& spec)
         cardBottomY = cardY + cardH;
         g_overlayCurrentKey.clear();
     } else {
-        cardY = vp_pos.y + vp_size.y * 0.15f;
         g_overlayCurrentKey = childId;
-        float ratioMaxY = vp_pos.y + vp_size.y * spec.cardBottomViewportRatio;
         auto it = g_overlayCardHeights.find(childId);
-        cardBottomY = (it != g_overlayCardHeights.end() && it->second > 0.0f)
+        const float measuredH = (it != g_overlayCardHeights.end()) ? it->second : 0.0f;
-                          ? std::min(cardY + it->second, ratioMaxY) : ratioMaxY;
+        if (measuredH > 0.0f) {
+            // Center the measured content; if it's taller than the window, anchor at the top margin.
+            cardY = (measuredH < vp_size.y - 32.0f)
+                        ? vp_pos.y + (vp_size.y - measuredH) * 0.5f
+                        : vp_pos.y + 16.0f;
+            cardBottomY = cardY + measuredH;
+        } else {
+            // First frame: content height not yet known. Anchor near the top; it re-centers next frame.
+            cardY = vp_pos.y + vp_size.y * 0.15f;
+            cardBottomY = vp_pos.y + vp_size.y * spec.cardBottomViewportRatio;
+        }
     }
     ImVec2 cardMin(cardX, cardY), cardMax(cardX + cardWidth, cardBottomY);