ci(release): upload updater .sig as release asset

tauri-action on macOS cross-target builds produces the .app.tar.gz.sig
locally but silently skips its upload ("Signature not found for the
updater JSON. Skipping upload..."), which caused generate-manifest to
abort on v1.34.15 because the .sig asset was absent.

New step after tauri-action locates the .sig under src-tauri/target/*/
release/bundle/macos/ and uploads it as Psysonic_aarch64.app.tar.gz.sig
or Psysonic_x64.app.tar.gz.sig — the exact filenames generate-update-
manifest.js expects.

Also bumps version to 1.34.16.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Psysonic",
-  "version": "1.34.15",
+  "version": "1.34.16",
   "identifier": "dev.psysonic.player",
   "build": {
     "beforeDevCommand": "npm run dev",