chore(release): bump version to 0.8.0-rc.4

[Nic-dorman]

Summary

• Bumps version in 4 files: `package.json`, `src-tauri/Cargo.toml`, `src-tauri/Cargo.lock`, `src-tauri/tauri.conf.json`
• Continues the 0.8.0 RC cycle (0.8.0 has not shipped stable yet)

Contents vs 0.8.0-rc.3

• Korean (ko) locale (feat(i18n): add Korean (ko) locale + extend CJK banner to ko #143) — machine-translated baseline, seventh supported language (en/ja/ko/nl/fr/bg/es)
• CJK font banner extended to ko — `한국어` prefix, same noto-cjk install commands

Test plan

• CI green on this bump PR
• After tag push: release CI builds installers (~22 min)
• Korean reviewer pulls rc.4 via in-app pre-release updater and verifies:
  • Settings → Language → `한국어` switches the UI
  • Strings render correctly across pages
  • On Linux: CJK banner shows with `한국어 /` prefix when noto-cjk is absent
• Existing rc.3 verification (CJK banner for ja, dropdown dark theme) remains valid

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