[armada877]

Let users choose the image mode.

• Add an images setting with auto, kitty, blocks, placeholder, and off, in ~/.config/syn/config.toml and in the SYN_IMAGES environment variable. SYN_IMAGES wins.
• auto does not change: syn uses kitty images when the terminal answers the kitty graphics query.
• Document a limitation: a terminal multiplexer that answers the kitty query itself, such as herdr, can report support even when the outer terminal (for example foot) cannot show kitty images. In that case, set images = "blocks".

Local checks passed: tests with local sockets, race tests, vet, formatting, build, and terminal checks of the modes, including foot with herdr.

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