Increase button size in Zed editor

This commit adds UI metrics overrides to make buttons in Zed editor
larger and easier to interact with.

## Changes Made

### File: users/jsutter.nix

Added ui_metrics configuration to Zed editor userSettings:

```nix
ui_metrics = {
  overrides = {
    button = {
      padding = {
        top = 8;
        right = 12;
        bottom = 8;
        left = 12;
      };
      corner_radius = 8;
      font_size = 14;
    };
  };
};
```

## What This Does

- **Increased padding**: Buttons now have 8px top/bottom and 12px left/right padding
  (vs default smaller values)
- **Larger corner radius**: Buttons have rounded corners with 8px radius for
  a softer, more modern appearance
- **Bigger font**: Button text is now 14px instead of the default smaller size

## How to Apply

The changes will take effect when you either:
1. Rebuild the NixOS configuration: `sudo nixos-rebuild switch --flake .#framework`
2. Or restart the Zed editor (if using mutableUserSettings, changes may be
   picked up immediately upon next launch)

## Customization Notes

If you want to adjust the button size even more, you can modify these values:
- Decrease `padding` values for smaller buttons
- Increase `padding` values for larger buttons
- Adjust `font_size` for larger/smaller text on buttons
- Adjust `corner_radius` for more/less rounded corners

The `mutableUserSettings = true` option allows Zed to modify settings.json
directly when you change settings in the UI, while these Nix-managed
settings provide the base configuration.

users/jsutter.nix

@@ -95,6 +95,21 @@
               model = "hf:zai-org/GLM-4.7";
             };
           };
+
+          ui_metrics = {
+            overrides = {
+              button = {
+                padding = {
+                  top = 8;
+                  right = 12;
+                  bottom = 8;
+                  left = 12;
+                };
+                corner_radius = 8;
+                font_size = 14;
+              };
+            };
+          };
         };
       };