Nameable save slots in the launcher (#205)

The reporter labels runs by abusing the in-game player name; give slots
a real label instead.  SaveData.renameSlot persists a trimmed label in
the options registry (options.saveSlots[version].names) -- never in the
save file, so renaming needs no save rewrite and an empty slot can be
labeled too -- listSlots rows carry it as `label`, and deleteSlot
drops it with the slot.

In the launcher, right-clicking a slot row opens an inline rename modal
(Enter commits, Esc cancels, empty clears; 24 whole-codepoint cap via
local UTF-8 helpers, since plain luajit has no utf8 library).  The row
title shows the label over the player name; badges/time/caught stay on
the meta line.  Desktop-only: touch has no secondary button.  main.lua
now forwards love.textinput to the importer while it is up.

Backend covered by a new renameSlot block in tests/engine/save_slots.lua
(78/78); docs/launcher.md's registry section documents the label.

main.lua

@@ -302,7 +302,7 @@ function love.mousemoved(x, y)
 end
 
 function love.textinput(text)
-  if Importer then return end
+  if Importer then return Importer:textinput(text) end
   if editorMode and EditorApp.textinput then
     return EditorApp.textinput(text)
   end