Signature actions enabled while account still loading

### What Happened?

When the account is still loading we get an empty signature dialog and buttons don't do anything

### Steps to Reproduce

1. Kill mail and restart it
2. Open the signatures dialog while mailboxes are still loading
3. Notice that buttons don't do anything until accounts are fully loaded

### Expected Behavior

We should have some kind of indication that things are still loading and disable any actions that won't work until loading has completed

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Early Access or Compiled from git

### Log Output

```shell

```

### Hardware Info

_No response_

Start by reproducing the issue in the signatures dialog while mailboxes are loading after restarting Mail. Trace the dialog's loading state and action handling; done means the dialog indicates loading and actions that depend on completed account loading remain disabled until loading finishes.

Written by the indexing model from the issue text.