Fix Forgejo CI: set empty signing password explicitly

Tauri requires TAURI_SIGNING_PRIVATE_KEY_PASSWORD to be defined,
even when the key has no password.

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

.forgejo/workflows/release.yml

@@ -35,6 +35,7 @@ jobs:
       - name: Build Tauri
         env:
           TAURI_SIGNING_PRIVATE_KEY: ${{ secrets.TAURI_SIGNING_PRIVATE_KEY }}
+          TAURI_SIGNING_PRIVATE_KEY_PASSWORD: ""
         run: |
           source "$HOME/.cargo/env"
           npx tauri build