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

CHANGELOG.md

@@ -5,6 +5,14 @@ All notable changes to PRAutoBlogger will be documented in this file.
 Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project uses [Semantic Versioning](https://semver.org/).
 
+## [0.13.9] - 2026-04-26
+
+### Fixed
+- Model picker sort by price now works correctly for image models — previously
+  \`applySort\` always read \`input_price_per_m\` (undefined on image models),
+  so every image had an effective price of 0 and price sort was a no-op.
+  Fix: detect image models via \`cost_per_image\` and use that field instead.
+
 ## [0.13.8] - 2026-04-26
 
 ### Added