feat(image-picker): thumbnails fill width at 6 per row

Fix the thumbnail grid to a constant 6 columns whose square cells scale to the
available width, instead of fixed 96px cells that left dead space on the right.

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

src/ui/windows/image_picker.h

@@ -207,12 +207,12 @@ public:
             if (!s_images.empty()) ImGui::Dummy(ImVec2(0, Layout::spacingSm()));   // gap before the thumbnails
         }
 
-        // Thumbnail grid.
+        // Thumbnail grid: a fixed 6 columns whose square cells scale to fill the width.
         {
             const float availW = ImGui::GetContentRegionAvail().x;
-            const float cell = 96.0f * dp;
             const float gap  = Layout::spacingSm();
-            const int cols = std::max(1, (int)((availW + gap) / (cell + gap)));
+            const int cols = 6;
+            const float cell = std::max(24.0f * dp, (availW - gap * (cols - 1)) / (float)cols);
             int col = 0;
             for (std::size_t i = 0; i < s_images.size(); ++i) {
                 if (col != 0) ImGui::SameLine(0, gap);