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

CLAUDE.md

@@ -396,9 +396,16 @@ When code changes are made to cm-dashboard, the NixOS configuration at `~/nixosb
      sha256 = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="; 
    }' 2>&1 | grep "got:"
    ```
+   
+   Example output:
+   ```
+   error: hash mismatch in fixed-output derivation '/nix/store/...':
+            specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
+               got:    sha256-x8crxNusOUYRrkP9mYEOG+Ga3JCPIdJLkEAc5P1ZxdQ=
+   ```
 
 4. **Update Configuration with Correct Hash**
-   Replace the placeholder with the hash from the error message.
+   Replace the placeholder with the hash from the error message (the "got:" line).
 
 5. **Commit NixOS Configuration**
    ```bash