fix+perf: audit fixes and GPU blur migration (v0.5.0)

Address all findings from quality, performance, and security audits:
- Filter greetd error descriptions consistently (security)
- Re-enable power buttons after failed action (UX bug)
- Narrow TOCTOU window in avatar loading via symlink_metadata (security)
- Allow @ in usernames for LDAP compatibility
- Eliminate unnecessary Vec allocation in passwd parsing
- Remove dead i18n field, annotate retained-for-future struct fields
- Fix if/if→if/else and noisy test output in power.rs

Replace CPU blur (image crate + disk cache + async orchestration) with
GPU blur via GskBlurNode — symmetric with moonlock and moonset.
Removes ~15 transitive dependencies and ~200 lines of caching code.

DECISIONS.md

@@ -1,6 +1,13 @@
 # Decisions
 
-## 2026-03-28 – Optional background blur via `image` crate
+## 2026-03-28 – GPU blur via GskBlurNode replaces CPU blur
+
+- **Who**: Ragnar, Dom
+- **Why**: CPU-side Gaussian blur (`image` crate) blocked the GTK main thread for 500ms–2s on 4K wallpapers at cold cache. Disk cache and async orchestration added significant complexity.
+- **Tradeoffs**: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper backgrounds. Removes `image` crate dependency entirely (~15 transitive crates eliminated). No disk cache needed.
+- **How**: `Snapshot::push_blur()` + `GskRenderer::render_texture()` on `connect_realize`. Blur happens once on the GPU when the widget gets its renderer, producing a concrete `gdk::Texture`. Zero startup latency. Symmetric with moonlock and moonset.
+
+## 2026-03-28 – Optional background blur via `image` crate (superseded)
 
 - **Who**: Selene, Dom
 - **Why**: Blurred wallpaper as greeter background is a common UX pattern for login screens