feat: app icon + Arch package (PKGBUILD + .desktop)

Add a desktop/taskbar identity and a system package for PeerSpeak.

- assets/icons/peerspeak.svg: master app icon — the in-app mic glyph
  over a P2P mesh of peer nodes, Catppuccin Mocha palette. Rendered to
  the hicolor raster sizes (16..512) committed alongside it.
- Window/taskbar icon: embed a 128x128 straight-RGBA blob and load it
  via iced from_rgba (keeps us off iced's heavy `image` feature). Set
  the Wayland/X11 app_id to "peerspeak" so compositors match the window
  to the .desktop launcher and show the icon natively.
- packaging/peerspeak.desktop: launcher (StartupWMClass=peerspeak).
- packaging/PKGBUILD: peerspeak-git VCS package — cargo --frozen build,
  --lib check, installs the binary, .desktop, and hicolor icons.

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

packaging/peerspeak.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=PeerSpeak
+GenericName=P2P Voice Chat
+Comment=Decentralized peer-to-peer voice chat
+Exec=peerspeak
+Icon=peerspeak
+Terminal=false
+Categories=Network;Telephony;Chat;
+Keywords=voice;chat;voip;p2p;peer;
+StartupWMClass=peerspeak