fix: address audit findings — blur channel mismatch, logout quit, config error logging

- Fix BGRA→RGBA channel swap in apply_blur so image::RgbaImage semantics
  match the actual pixel data from GDK texture download
- Logout now calls app.quit() like lock does, via new quit_after field on
  ActionDef (replaces fragile magic string comparison)
- Log TOML parse errors to stderr instead of silently ignoring
- Remove pointless zlib compression of JPEG wallpaper in GResource
- Add tests for quit_after behavior and config error handling

DECISIONS.md

@@ -2,6 +2,13 @@
 
 Architectural and design decisions for Moonset, in reverse chronological order.
 
+## 2026-03-28 – Replace action name dispatch with `quit_after` field
+
+- **Who**: Hekate, Dom
+- **Why**: Post-action behavior (quit the app or not) was controlled by comparing `action_name == "lock"` — a magic string duplicated from the action definition. Renaming an action would silently break the dispatch.
+- **Tradeoffs**: Adds a field to `ActionDef` that most actions set to `false`. Acceptable because it makes the contract explicit and testable.
+- **How**: `ActionDef.quit_after: bool` — `true` for lock and logout, `false` for hibernate/reboot/shutdown.
+
 ## 2026-03-28 – Optional background blur via `image` crate
 
 - **Who**: Hekate, Dom