fix: address audit findings — security, performance, and correctness

- Use absolute paths for all binaries in power.rs to prevent PATH hijacking
- Implement POWER_TIMEOUT via try_wait() polling (was declared but unused)
- Fix potential panic in load_background_texture when GResource path
  fails to_str() — now falls back to known wallpaper resource path
- Compress wallpaper.jpg in GResource bundle (saves ~374 KB in binary)
- Merge double idle_add_local_once into single cycle for faster focus
- Centralize GRESOURCE_PREFIX as pub(crate) const in main.rs
- Fix fallback user UID from 0 (root) to u32::MAX
- Fix CSS comment: "square card" → "circular card" (border-radius: 50%)

resources/resources.gresource.xml

@@ -2,7 +2,7 @@
 <gresources>
   <gresource prefix="/dev/moonarch/moonset">
     <file>style.css</file>
-    <file>wallpaper.jpg</file>
+    <file compressed="true">wallpaper.jpg</file>
     <file>default-avatar.svg</file>
   </gresource>
 </gresources>