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>

assets/icons/peerspeak.svg

@@ -0,0 +1,41 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="256" height="256" viewBox="0 0 256 256" xmlns="http://www.w3.org/2000/svg">
+  <!-- PeerSpeak app icon: in-app mic glyph + P2P mesh nodes, Catppuccin Mocha. -->
+  <defs>
+    <linearGradient id="tile" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#1e1e2e"/>
+      <stop offset="1" stop-color="#181825"/>
+    </linearGradient>
+  </defs>
+
+  <!-- Rounded-square tile -->
+  <rect x="20" y="20" width="216" height="216" rx="48" fill="url(#tile)"
+        stroke="#313244" stroke-width="3"/>
+
+  <!-- P2P mesh: edges (under nodes + mic) -->
+  <g stroke="#45475a" stroke-width="6" stroke-linecap="round" fill="none">
+    <line x1="74"  y1="74"  x2="128" y2="128"/>
+    <line x1="182" y1="74"  x2="128" y2="128"/>
+    <line x1="74"  y1="182" x2="128" y2="128"/>
+    <line x1="182" y1="182" x2="128" y2="128"/>
+    <line x1="74"  y1="74"  x2="182" y2="74"/>
+    <line x1="74"  y1="182" x2="182" y2="182"/>
+  </g>
+
+  <!-- P2P mesh: peer nodes -->
+  <g fill="#b4befe">
+    <circle cx="74"  cy="74"  r="11"/>
+    <circle cx="182" cy="74"  r="11"/>
+    <circle cx="74"  cy="182" r="11"/>
+    <circle cx="182" cy="182" r="11"/>
+  </g>
+
+  <!-- Microphone (hero) — same geometry as the in-app Mic icon, scaled 6.4x -->
+  <g fill="none" stroke="#89b4fa" stroke-width="13"
+     stroke-linecap="round" stroke-linejoin="round">
+    <rect x="108.8" y="68.8" width="38.4" height="70.4" rx="19.2"/>
+    <path d="M 169.6 123.2 A 41.6 41.6 0 0 0 86.4 123.2"/>
+    <line x1="128" y1="164.8" x2="128" y2="187.2"/>
+    <line x1="105.6" y1="187.2" x2="150.4" y2="187.2"/>
+  </g>
+</svg>