Since #8999, `macos-custom-icon` works when its a fully expanded
absolute path like `/Users/username/dir/icon.icns`, but not when it's
abbreviated as `~/dir/icon.icns`. Users were understandably surprised
and confused by this. This PR adds tilde expansion using `NSString`s
built-in property for this.

Also removed a line from the config docs that seemed erroneous. Given
that the option has a functional default, it seems incorrect to say that
it's required.

Fixes #9016

Changing `supportedModes` to `background` seems to have fixed #8669.

> Debugging AppIntents with Xcode is a pain. I had to delete all the
local builds to make it take effect. The build product of `zig` might
cause confusion if none of your changes reflect in the Shortcuts app.
There were too many ghosts on my computer. 👻👻👻

- Tahoe


https://github.com/user-attachments/assets/88d0d567-edf5-4a7e-b0a3-720e50053746

- Sequoia 


https://github.com/user-attachments/assets/a77f1431-ca92-4450-bce9-5f37ef232d4f

Fixes #8713

This stores the last closed state of the quick terminal by screen
pointer. We use a weak mapping so if a screen is unplugged we'll clear
the memory. We will not remember the size if you unplug and replug in a
monitor.

Fixes #8734

This forces the app icon to be set on another event loop tick from the
main startup.

In the future, we should load and set the icon completely in another
thread. It appears that all the logic we have is totally thread-safe.

Fixes #8785

This is the callback AppKit sends when it wants to know if our
application can handle sending and receiving certain types of data.

The prior implementaiton was incorrect and would erroneously claim
support over combinations that we couldn't handle (at least, at the
SurfaceView layer).

This corrects the implementation. The services we expect still show up
and the error in 8785 goes away.

Fixes #8783

Our new tab flow will never have a previously focused window because its
triggered by a service so we need to use the "preferred parent" logic we
have to open this in the last focused window.

This was a regression we didn't fix before 1.2.