Improve widget formatting and add logged-in users support

Services widget:
- Fix disk quota formatting with proper rounding instead of truncation
- Remove decimals from RAM quotas and use GB instead of G
- Change quota display to use GB consistently

Backups widget:
- Change GiB to GB for consistency
- Remove spaces between numbers and units
- Update disk usage format to match other widgets: used (totalGB)
- Remove percentage display for cleaner format

System widget:
- Add support for logged-in users in description lines
- Format C-states with "C-State:" prefix on first line, indent subsequent lines
- Add logged_in_users field to SystemSummary data structure

Documentation:
- Add example hash error output to NixOS update instructions

README.md

@@ -486,8 +486,15 @@ nix-build --no-out-link -E 'with import <nixpkgs> {}; fetchFromGitea {
 }' 2>&1 | grep "got:"
 ```
 
+Example output:
+```
+error: hash mismatch in fixed-output derivation '/nix/store/...':
+         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
+            got:    sha256-x8crxNusOUYRrkP9mYEOG+Ga3JCPIdJLkEAc5P1ZxdQ=
+```
+
 #### 4. Update the Hash
-Replace the placeholder with the correct hash:
+Replace the placeholder with the correct hash from the error message (the "got:" line):
 ```nix
 hash = "sha256-vjy+j91iDCHUf0RE43anK4WZ+rKcyohP/3SykwZGof8=";  # Use actual hash
 ```