feat: add interactive configuration menu

Running without a GIF argument (or with -i/--interactive) now launches a
questionary-based menu that selects a source GIF from the current directory
and configures every conversion parameter before running. Passing a GIF on
the command line behaves exactly as before.

- extract a unified run_conversion(ConversionConfig) pipeline shared by the
  CLI and the menu, removing the duplicated convert loop in main()
- make the gif positional optional and add -i/--interactive
- import questionary lazily inside the menu so plain CLI usage gains no hard
  dependency, with a clear install hint when it is missing
- add __version__ / --version (0.2.0) and document the menu in README/AGENTS

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

requirements.txt

@@ -1 +1,2 @@
 Pillow
+questionary