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/README.md

@@ -12,7 +12,7 @@ runtime, so there are no extra DLLs to bundle. The installer payload is just the
 ## Version compatibility
 
 The installer version tracks the crate version in `Cargo.toml` (currently
-**0.3.0**) — keep `MyAppVersion` in `peerspeak.iss` in sync when it changes.
+**0.4.0**) — keep `MyAppVersion` in `peerspeak.iss` in sync when it changes.
 
 Per `VERSIONING.md`, a **MINOR** bump in `0.x` is a **breaking wire change**:
 peers on different MINOR versions can't connect (they fail fast at the