fix: use /dev/tty for width autodetect; default config width: 0

[HeikoSchlittermann]

Two related width-autodetect fixes.

1. Prefer /dev/tty over os.Stdout for terminal-size detection.

term.GetSize(os.Stdout.Fd()) fails whenever stdout isn't a TTY — e.g. glow x.md | less, glow x.md > out, or some glow <x.md setups — and glow silently falls back to 80 columns. /dev/tty refers to the controlling terminal regardless of how the standard fds are wired, so it works in all those cases. Falls back to the original term.GetSize(os.Stdout.Fd()) path when /dev/tty can't be opened, so behaviour is a strict superset.

2. Default config now ships width: 0 instead of width: 80.

glow config writes a hard-coded YAML template on first run with width: 80, which silently overrides the documented 0 = autodetect default (viper.SetDefault("width", 0) in main.go). The auto-generated config is therefore strictly more restrictive than no config at all. Aligning the template with SetDefault restores the documented behaviour.

Existing users with the old config still get 80 until they edit it once; new users get autodetect out of the box.

[HeikoSchlittermann]

Related: #952 tackles a third aspect of the same area — removing the 120-column cap on the auto-detected width. The two PRs are orthogonal (this one fixes which fd is queried and the default-config template; #952 fixes the cap applied afterwards) and could be merged independently or together.