Remove AppImage from bundle targets

Restrict tauri.conf.json bundle targets to deb/rpm so `cargo tauri
build` no longer produces an AppImage (the README's AppImage mentions
were already dropped in the prior commit).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src-tauri/tauri.conf.json

@@ -25,7 +25,7 @@
   },
   "bundle": {
     "active": true,
-    "targets": "all",
+    "targets": ["deb", "rpm"],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",