chore(packaging): bump Windows installer to 0.4.0

The 0.4.0 Windows binary was cross-built (x86_64-pc-windows-gnu,
build-std static) and verified in the libvirt Windows 11 VM: the raw
exe launches/renders the full v0.4.0 UI and runs stably, and the
compiled installer was test-installed end-to-end (Program Files exe
sha256 1a211eb6…, Start-menu shortcut, firewall rule, launch from
the installed location) before publishing to the v0.4.0 release.

- peerspeak.iss: MyAppVersion 0.3.0 -> 0.4.0 (installer output is
  peerspeak-0.4.0-setup.exe)
- INSTALL.md / README.md: update the 0.3.0 filename/version references

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/windows/INSTALL.md

@@ -8,7 +8,7 @@ it once, then you and I connect directly to each other.
 
 ## 1. Install it
 
-1. Double-click **`peerspeak-0.3.0-setup.exe`** (the file I sent you).
+1. Double-click **`peerspeak-0.4.0-setup.exe`** (the file I sent you).
 
 2. **Windows will probably show a blue "Windows protected your PC" warning.**
    This is normal — it shows up for any app that isn't from a big company with a