feat: add optional background blur, align to shared texture pattern

Gaussian blur applied at texture load time when `background_blur` is
set in moonlock.toml. Refactored wallpaper loading from per-window
Picture::for_filename() to shared gdk::Texture pattern (matching
moonset/moongreet), avoiding redundant JPEG decoding on multi-monitor.

DECISIONS.md

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+# Decisions
+
+Architectural and design decisions for Moonlock, in reverse chronological order.
+
+## 2026-03-28 – Optional background blur via `image` crate
+
+- **Who**: Nyx, Dom
+- **Why**: Consistent with moonset/moongreet — blurred wallpaper as lockscreen background is a common UX pattern
+- **Tradeoffs**: Adds `image` crate dependency (~15 transitive crates); CPU-side Gaussian blur at load time adds startup latency proportional to image size and sigma. Acceptable because blur runs once and the texture is shared across monitors.
+- **How**: `load_background_texture(bg_path, blur_radius)` loads texture, optionally applies `imageops::blur()`, returns `gdk::Texture`. Config option `background_blur: Option<f32>` in TOML.
+
+## 2026-03-28 – Shared wallpaper texture pattern (aligned with moonset/moongreet)
+
+- **Who**: Nyx, Dom
+- **Why**: Previously loaded wallpaper per-window via `Picture::for_filename()`. Multi-monitor setups decoded the JPEG redundantly. Blur feature requires texture pixel access anyway.
+- **Tradeoffs**: Slightly more code in main.rs (texture loaded before window creation), but avoids redundant decoding and enables the blur feature.
+- **How**: `load_background_texture()` in lockscreen.rs decodes once, `create_background_picture()` wraps shared `gdk::Texture` in `gtk::Picture`. Same pattern as moonset/moongreet.