feat(icon): app icon with embedded PNG, Windows .ico and resource objects

One SVG source (assets/icon/talk.svg: speech bubble with level bars).
assets/icon/gen.go renders it with inkscape into the embedded PNG
(internal/icon, installed next to the config for notifications), the
multi-size talk.ico and the COFF .rsrc objects for windows/amd64 and
arm64 that give talk.exe its icon. The generator writes the resource
objects itself, so no resource compiler is needed; everything it
produces is committed and a plain go build picks it up.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

assets/icon/talk.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256" width="256" height="256">
+  <path fill="#0F6E86" d="M56 28h144c22 0 40 18 40 40v96c0 22-18 40-40 40H112l-48 36c-6 5-16 1-16-8v-28H56c-22 0-40-18-40-40V68c0-22 18-40 40-40z"/>
+  <g fill="#FFFFFF">
+    <rect x="76" y="100" width="24" height="56" rx="12"/>
+    <rect x="116" y="72" width="24" height="112" rx="12"/>
+    <rect x="156" y="88" width="24" height="80" rx="12"/>
+  </g>
+</svg>