chore: migrate omarchy state from ~/.config to ~/.local/state

Move all omarchy runtime state (themes, backgrounds, current theme)
from ~/.config/omarchy/current to ~/.local/state/omarchy/current
to keep the config dir free of generated files. Add mise task for
recreating symlinks and update credential helpers for tea.

script/omarchy-theme-bg-set

@@ -10,7 +10,7 @@ if [[ -z $1 ]]; then
 fi
 
 BACKGROUND="$(realpath "$1")"
-CURRENT_BACKGROUND_LINK="$HOME/.config/omarchy/current/background"
+CURRENT_BACKGROUND_LINK="$HOME/.local/state/omarchy/current/background"
 
 if [[ ! -f "$BACKGROUND" ]]; then
   echo "File does not exist: $BACKGROUND" >&2