Add Windows installer (Inno Setup) + GUI-subsystem release builds

Package PeerSpeak for Windows as a single self-contained binary. The GUI
icon, notification chimes, and avatar presets are already embedded via
include_bytes!, and the cross-compiled .exe is statically linked (no extra
DLLs), so the installer payload is just peerspeak.exe plus an .ico.

- src/main.rs: set windows_subsystem = "windows" for release builds so the
  GUI launches without a stray console window (debug keeps the console for
  stderr/panics).
- packaging/windows/: Inno Setup script (peerspeak.iss), multi-resolution
  app icon (peerspeak.ico), and a build README. The installer drops a
  Start-menu/desktop shortcut, optionally adds a Windows Firewall allow-rule
  (iroh UDP hole-punching), and provides an uninstaller.
- win-cross-build.sh: promote the cross-build helper from a throwaway to the
  documented installer build step; .gitignore the staged exe + compiled
  setup.exe build artifacts.

Built with Inno Setup 6.7.1 under Wine; binary is unsigned (SmartScreen will
warn until code-signed).

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

src/main.rs

@@ -1,3 +1,7 @@
+// On Windows, suppress the extra console window for release GUI builds while
+// keeping it in debug builds so stderr/panics stay visible during development.
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
 fn main() {
     if let Err(e) = peerspeak::app::run_gui() {
         eprintln!("Error running GUI: {:?}", e);