feat: cost breakdown popover on post list (v0.4.7)

Cost column is now clickable — hovering shows a per-step breakdown
with stage name, model, token count, and cost for every LLM call.
Pure CSS popover, no JS dependency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,13 @@ 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/).
 
+## [0.4.7] — 2026-04-19
+
+### Added
+- **Cost breakdown popover.** The Cost column in the post list is now clickable.
+  Hovering or clicking shows a per-step breakdown: stage name, model used, token
+  count, and cost for every LLM call that produced the article.
+
 ## [0.4.6] — 2026-04-19
 
 ### Added