fix(theme): exit on missing theme; persist bg picks across rebuilds

Two related bugs in the theme switcher scripts:

(1) nomarchy-theme-set printed a warning when the theme directory didn't
exist but kept going — it wrote the bad name into state.json and ran
nomarchy-env-update on a broken state. Added an exit 1 after the
warning.

(2) nomarchy-theme-bg-set updated the live ~/.config/nomarchy/current/
background symlink + restarted swaybg but never wrote state.json. The
script is called by the walker background-selector menu
(elephant/nomarchy_background_selector.lua) and by the nomarchy-
wallpaper CLI wrapper, so every wallpaper picked via either path
silently reverted to the active theme's default on the next
home-manager switch — themes/engine/files.nix re-resolves
config.nomarchy.wallpaper at every rebuild. Now writes the chosen path
into state.json's wallpaper field, mirroring nomarchy-theme-bg-next.
Also added a file-exists check so a bogus path fails loudly instead of
leaving a dangling symlink + a failed swaybg process.

themes/engine/scripts/nomarchy-theme-bg-set

@@ -1,7 +1,12 @@
 #!/bin/bash
 set -e
 
-# Sets the specified image as the current background
+# Sets the specified image as the current background.
+# Updates state.json so the choice survives the next rebuild — without
+# that write, themes/engine/files.nix re-resolves the wallpaper from
+# `nomarchy.wallpaper` on the next home-manager switch and silently
+# falls back to the active theme's default background, undoing the
+# user's pick.
 
 if [[ -z $1 ]]; then
   echo "Usage: nomarchy-theme-bg-set <path-to-image>" >&2
@@ -9,7 +14,22 @@ if [[ -z $1 ]]; then
 fi
 
 BACKGROUND="$1"
-CURRENT_BACKGROUND_LINK="$HOME/.config/nomarchy/current/background"
+
+if [[ ! -f "$BACKGROUND" ]]; then
+  echo "Background image not found: $BACKGROUND" >&2
+  exit 1
+fi
+
+STATE_DIR="$HOME/.config/nomarchy"
+STATE_FILE="$STATE_DIR/state.json"
+CURRENT_BACKGROUND_LINK="$STATE_DIR/current/background"
+
+mkdir -p "$STATE_DIR"
+[[ ! -f $STATE_FILE ]] && echo "{}" > "$STATE_FILE"
+
+# Persist the choice for the next rebuild.
+TMP_JSON=$(mktemp)
+jq --arg wp "$BACKGROUND" '.wallpaper = $wp' "$STATE_FILE" > "$TMP_JSON" && mv "$TMP_JSON" "$STATE_FILE"
 
 # Create symlink to the new background
 ln -nsf "$BACKGROUND" "$CURRENT_BACKGROUND_LINK"

themes/engine/scripts/nomarchy-theme-set

@@ -30,7 +30,8 @@ mkdir -p "$STATE_DIR"
 [[ ! -f $STATE_FILE ]] && echo "{}" > "$STATE_FILE"
 
 if [ ! -d "$THEMES_DIR/$THEME_NAME" ]; then
-  echo "Theme '$THEME_NAME' not found in $THEMES_DIR"
+  echo "Theme '$THEME_NAME' not found in $THEMES_DIR" >&2
+  exit 1
 fi
 
 TMP_JSON=$(mktemp)