Leaderboard: clear button and hygiene for migrated legacy data

A scratch test earlier in development wrote a fake 42s Beginner record into
the legacy ~/.config/minesweeper/leaderboard.ini, which the migration imported;
as a stored best it can never be beaten by slower real games, so the popover
showed a lower time than actually played.

- Add a 'Clear scores' button to the leaderboard popover
- Delete the legacy file after a successful migration so stale data can't
  be re-imported

src/leaderboard.cpp

@@ -3,6 +3,7 @@
 #include <glibmm/keyfile.h>
 #include <glibmm/miscutils.h>
 #include <glib.h>
+#include <glib/gstdio.h>
 
 namespace {
 std::string file_path() {
@@ -19,9 +20,11 @@ void Leaderboard::load() {
     auto path = file_path();
     auto legacy = Glib::build_filename(
         Glib::get_user_config_dir(), "minesweeper", "leaderboard.ini");
+    bool migrated = false;
     if (!Glib::file_test(path, Glib::FileTest::EXISTS) &&
         Glib::file_test(legacy, Glib::FileTest::EXISTS)) {
         path = legacy;
+        migrated = true;
     }
 
     try {
@@ -32,6 +35,11 @@ void Leaderboard::load() {
                 times_[group.raw()] = kf->get_integer(group, "best");
             }
         }
+        if (migrated) {
+            // Stale legacy files can hold outdated/experimental data; drop it
+            // so it can never be re-imported after the current file is gone.
+            g_remove(legacy.c_str());
+        }
     } catch (const Glib::Error&) {
         // No file yet or unreadable — start with an empty leaderboard
     }
@@ -52,6 +60,12 @@ std::optional<int> Leaderboard::best_time(const std::string& difficulty) const {
     return it->second;
 }
 
+void Leaderboard::clear() {
+    load();
+    times_.clear();
+    save();
+}
+
 void Leaderboard::save() {
     auto kf = Glib::KeyFile::create();
     for (const auto& [difficulty, seconds] : times_) {