Fix desktop CI: add missing files and fix workflow errors

[vincentmakes]

• Add desktop/package-lock.json (required by npm ci)
• Generate Tauri icons from existing icon.png
• Replace deprecated macos-13 runner with macos-latest
• Split tauri-action into release (tag push) and test (workflow_dispatch) steps
• Add desktop/node_modules to .gitignore

https://claude.ai/code/session_01UvYKdrhaLpgAVwWrGJTvGj

Description

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (changes to docs only — no version bump needed)
• Translation (new or updated language files)
• Refactoring (no functional changes)

Checklist

Required for all code changes

• I have tested my changes locally (npm test passes)
• Version has been bumped in all 3 files (package.json, package-lock.json, version.json)
• CHANGELOG.md has been updated with a new entry under the correct version

If adding or changing user-visible strings

• No hardcoded English — all strings use t('key') in JS or data-i18n in HTML
• New i18n keys added to en.json and all 7 other locale files (de, fr, nl, es, it, pt, zh)
• escapeHtml() used for any user-provided content rendered as HTML

If documentation-only change

• No version bump included (docs changes must not bump the version)

Screenshots (if applicable)