Fix app icons: regenerate as RGBA and add macOS .icns

The bundled .ico had non-RGBA PNGs which caused Tauri's macOS bundler
to fail with "The PNG is not in RGBA format!". Regenerated all icons
from the source PNG as proper RGBA, and added icon.icns for macOS.

Also fixed bundle identifier from "com.localtranscription.app" (the
.app suffix conflicts with macOS bundle extension) to
"net.anhonesthost.local-transcription".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "productName": "Local Transcription",
   "version": "1.4.1",
-  "identifier": "com.localtranscription.app",
+  "identifier": "net.anhonesthost.local-transcription",
   "build": {
     "frontendDist": "../dist",
     "devUrl": "http://localhost:1420",
@@ -30,6 +30,7 @@
       "icons/32x32.png",
       "icons/128x128.png",
       "icons/128x128@2x.png",
+      "icons/icon.icns",
       "icons/icon.ico",
       "icons/icon.png"
     ]