Add titles-only list export and Custom lists

Export
- GET /l/<list>/export?format=csv|txt downloads a list's titles in rank
  order, one per line, with no header or other fields (CSV gets a UTF-8
  BOM for Excel)
- Export menu in the list editor and, for the owner, on the public list
  page; hidden lists stay owner-only

Custom lists
- New "Custom" category for anything not built in (sports teams,
  drinks…): owner-named, slug from the title, entries added by hand or
  from an unfiltered Wikipedia search
- Per-list artwork shape (square or tall), stored in a new lists.art
  column (migration 2)
- Art can be uploaded while adding an entry, not only after

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/categories.go

@@ -19,6 +19,9 @@ var Categories = []Category{
 	{Key: "song", Label: "Songs", Singular: "Song", Icon: "music", Aspect: "square", SubtitleHint: "Artist"},
 	{Key: "game", Label: "Video Games", Singular: "Video Game", Icon: "gamepad", Aspect: "poster", SubtitleHint: "Developer"},
 	{Key: "theater", Label: "Theater", Singular: "Show", Icon: "masks", Aspect: "poster", SubtitleHint: "Creators"},
+	// Custom lists hold anything else (sports teams, drinks…); the owner
+	// names the list and adds entries by hand or from Wikipedia.
+	{Key: "custom", Label: "Custom", Singular: "Favorite", Icon: "star", Aspect: "square", SubtitleHint: "Subtitle"},
 }
 
 func CategoryByKey(key string) (Category, bool) {