feat(branding): bundle 64x64 icon in tauri.conf

Follow-up to PR #186 review: tauri icon CLI generates 64x64.png
but it was not declared in bundle.icon, so packagers (deb/rpm)
weren't picking it up. Add it alongside the other Linux sizes.

src-tauri/tauri.conf.json

@@ -26,6 +26,7 @@
     "targets": ["nsis", "deb", "rpm"],
     "icon": [
       "icons/32x32.png",
+      "icons/64x64.png",
       "icons/128x128.png",
       "icons/128x128@2x.png",
       "icons/icon.icns",