perf(image-picker): cull off-screen thumbnail cells

The image picker laid out and drew every cell in the directory and image
grids, including the many cells scrolled out of view.

- A cell whose rectangle is not visible now reserves the same space with
  a dummy of the cell size, so the scroll extent is unchanged, and skips
  the hit-test and draw.

No visible change; only off-screen cells are skipped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/image_picker.h

@@ -194,6 +194,12 @@ public:
                 if (col != 0) ImGui::SameLine(0, cellGap);
                 ImVec2 mn = ImGui::GetCursorScreenPos();
                 ImVec2 mx(mn.x + cellW, mn.y + cellH);
+                // Off-screen cell: reserve the same space (scroll extent unchanged) but skip hit-test/draw.
+                if (!ImGui::IsRectVisible(mn, mx)) {
+                    ImGui::Dummy(ImVec2(cellW, cellH));
+                    col = (col + 1) % cols;
+                    continue;
+                }
                 ImGui::PushID((int)(i + 1));
                 const bool clicked = ImGui::InvisibleButton("##idir", ImVec2(cellW, cellH));
                 const bool hov = ImGui::IsItemHovered();
@@ -224,6 +230,12 @@ public:
                 if (col != 0) ImGui::SameLine(0, gap);
                 ImVec2 mn = ImGui::GetCursorScreenPos();
                 ImVec2 mx(mn.x + cell, mn.y + cell);
+                // Off-screen cell: reserve the same space (scroll extent unchanged) but skip hit-test/draw.
+                if (!ImGui::IsRectVisible(mn, mx)) {
+                    ImGui::Dummy(ImVec2(cell, cell));
+                    col = (col + 1) % cols;
+                    continue;
+                }
                 const std::string abs = (std::filesystem::path(s_dir) / s_images[i]).string();
                 const bool sel = (s_selected == abs);
                 ImGui::PushID((int)(i + 1));