fix: model picker price sort broken for image models (v0.13.9)

applySort() always read input_price_per_m for price sorting, but image
models carry cost_per_image instead — input_price_per_m is undefined on
them, so every image model got an effective price of 0 and the sort was a
no-op.

Fix: detect image models via cost_per_image !== undefined and use that
field when present; fall back to input_price_per_m for text models.

Files changed:
- assets/js/model-picker.js — applySort price branch
- prautoblogger.php        — version bump 0.13.8 → 0.13.9
- CHANGELOG.md             — release notes

prautoblogger.php

@@ -9,7 +9,7 @@
  * Plugin Name:       PRAutoBlogger
  * Plugin URI:        https://peptiderepo.com/prautoblogger
  * Description:       Monitors social media for trending topics, generates SEO-friendly blog posts using AI, and publishes them on a daily schedule with full cost tracking and self-improvement metrics.
- * Version:           0.13.8
+ * Version:           0.13.9
  * Requires at least: 6.0
  * Requires PHP:      7.4
  * Author:            PeptideRepo
@@ -35,7 +35,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 | Defined here so every file in the plugin can reference paths, versions,
 | and limits without magic strings.
 */
-define( 'PRAUTOBLOGGER_VERSION', '0.13.8' );
+define( 'PRAUTOBLOGGER_VERSION', '0.13.9' );
 define( 'PRAUTOBLOGGER_DB_VERSION', '1.1.0' );
 define( 'PRAUTOBLOGGER_PLUGIN_FILE', __FILE__ );
 define( 'PRAUTOBLOGGER_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );