chore(release): bump version to 0.8.0

[Nic-dorman]

Summary

• Promotes `0.8.0-rc.4` to stable `0.8.0`
• First stable in the 0.8 line
• Bumps version in 4 files: `package.json`, `src-tauri/Cargo.toml`, `src-tauri/Cargo.lock`, `src-tauri/tauri.conf.json`

Contents vs 0.7.1 (last stable)

• i18n Phase 1 — vue-i18n framework + seven locales (en/ja/ko/nl/fr/bg/es)
• CJK font install banner on Linux (feat(i18n): add CJK font install banner for Linux #139) — persistent dismissible banner shown when locale is in the CJK set and `noto-cjk` is likely missing
• Native select popups follow dark theme on Linux (fix(theme): make native select popups follow dark theme on Linux #141) — `color-scheme` on `:root` fixes GTK-themed light dropdown popups
• ant-core 0.2.3 → 0.2.5 — merkle PUT timeout alignment + spill-dir leak fix on SIGKILL/OOM

Test plan

• Linux tester verified CJK font banner on `ja` locale (rc.3)
• Dropdown dark-mode fix verified (rc.4)
• Korean tester verified the `ko` locale (rc.4)
• CI green on this bump PR
• After tag push: release CI builds stable installers (~22 min)
• Custom release notes applied via `gh release edit v0.8.0 --notes-file .planning/release-notes-0.8.0.md` once release exists
• Stable installer pulled via in-app updater (without pre-release toggle)

🤖 Generated with Claude Code