Align --list-images output on a tab stop

Pad each name:tag with enough tabs (not spaces) to reach one 8-column
tab stop past the longest entry in the list, so filenames line up
regardless of how much shorter other names/tags are.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main.cpp

@@ -14,6 +14,7 @@
 // with this program; if not, write to the Free Software Foundation, Inc.,
 // 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
+#include <algorithm>
 #include <array>
 #include <cstdlib>
 #include <filesystem>
@@ -216,8 +217,25 @@ int list_images_command(const std::filesystem::path& dir) {
     if (!images) {
         return 1;
     }
+
+    std::vector<std::string> refs;
+    refs.reserve(images->size());
+    size_t max_len = 0;
     for (const auto& ref : *images) {
-        fmt::print("{}:{}\t{}\n", ref.name, ref.tag, ref.path.filename().string());
+        refs.push_back(fmt::format("{}:{}", ref.name, ref.tag));
+        max_len = std::max(max_len, refs.back().size());
+    }
+
+    // Pad each entry with tabs (not spaces) so filenames line up on an 8-column tab
+    // stop past the longest name:tag, however long that is.
+    constexpr size_t kTabWidth = 8;
+    size_t target_tabs = max_len / kTabWidth + 1;
+
+    for (size_t i = 0; i < images->size(); ++i) {
+        size_t tabs_used = refs[i].size() / kTabWidth;
+        size_t tabs_needed = target_tabs > tabs_used ? target_tabs - tabs_used : 1;
+        fmt::print("{}{}{}\n", refs[i], std::string(tabs_needed, '\t'),
+                   (*images)[i].path.filename().string());
     }
     return 0;
 }