fix: add play mode toggle, wire play history API, add back navigation buttons

- Add play mode cycle button (seq/loop/random/rloop) to Player bar
- Wire GET /api/player/:botId/history to query database instead of returning empty array
- Pass database instance from server.ts to createPlayerRouter
- Add back navigation button to Playlist, Lyrics, History, Search, and Settings views
- SongCard already had dblclick-to-play (no change needed)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/src/views/Playlist.vue

@@ -1,5 +1,9 @@
 <template>
   <div class="playlist-page">
+    <button class="back-btn" @click="$router.back()">
+      <Icon icon="mdi:arrow-left" />
+      返回
+    </button>
     <div v-if="loading" class="loading">加载中...</div>
 
     <template v-else-if="playlist">
@@ -101,6 +105,17 @@ onMounted(async () => {
 </script>
 
 <style lang="scss" scoped>
+.back-btn {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 14px;
+  opacity: 0.7;
+  margin-bottom: 16px;
+  transition: opacity var(--transition-fast);
+  &:hover { opacity: 1; }
+}
+
 .playlist-hero {
   display: flex;
   gap: 32px;