[Feature request] Make insert feedback corner radius and stroke width configurable

The insert/drag feedback overlay is drawn with a hardcoded corner radius. From `src/view.c`:

```c
#define INSERT_FEEDBACK_WIDTH 2
#define INSERT_FEEDBACK_RADIUS 9
```

Currently only the color is exposed, via `insert_feedback_color`.

Window corner radii on macOS are no longer a fixed target:

- macOS 26 Tahoe uses a radius that varies with toolbar height, producing several different values across apps
- macOS 27 Golden Gate standardizes windows at 20pt
- Users can override the system radius entirely with `defaults write -g NSConvolutionOverride1 -float `

In all three cases the fixed 9pt feedback radius no longer matches the windows it is previewing, which looks rather inconsistent. This is most noticeable for anyone who has set a square or near-square radius, where a rounded overlay sits on top of hard-cornered windows.

So I thought adding config options mirroring the existing `insert_feedback_color` would be great:

```
yabai -m config insert_feedback_radius 6
yabai -m config insert_feedback_width 2
```

Keeping 9 and 2 as the defaults would preserve current behavior for everyone else.

While you could patch `src/view.c` and build from source, it means giving up the Homebrew install path for a two-token change.

Thanks a lot for maintaining this!
Yabai is great and I couldn't imagine myself using macos without it.

Start in src/view.c at INSERT_FEEDBACK_WIDTH and INSERT_FEEDBACK_RADIUS, then trace how the existing insert_feedback_color option is exposed and applied. Add configurable radius and stroke width with defaults of 9 and 2, and verify that the yabai config commands change the overlay while preserving those defaults.

Written by the indexing model from the issue text.