feat(i18n): add Korean as a selectable UI language

[siakun]

Summary

Adds Korean (ko) as a fourth UI language, alongside 简体中文, 繁體中文 and English.

src/i18n/ko.json is a complete translation of en.json: all 916 keys, none
added or dropped, and every {{interpolation}} placeholder identical in name and
count to the English source. Key order matches en.json, so the two files stay
easy to diff against each other as strings are added.

The wiring is small:

• src/i18n/index.ts — ko added to resources and SUPPORTED_LANGUAGES
• src/views/Settings.tsx — 한국어 added to the language picker
• scripts/prepare-release.mjs — ko.json added to the files whose
  settings.version is bumped, so the version shown in About cannot drift away
  from package.json

One decision worth a maintainer's eye: fallbackLng

fallbackLng changes from the string "zh" to a per-language map:

fallbackLng: { ko: ["en", "zh"], default: ["zh"] }

zh, zh-TW and en keep exactly the behaviour they have today — they still
fall back to zh. Only ko differs: it tries English first.

The reason is what happens in the window between a feature landing and its
translation catching up. zh-TW.json is currently missing some keys and falls
back to zh, which works out fine — a reader of Traditional Chinese can act on
Simplified Chinese. A Korean reader cannot, so an untranslated key is strictly
more useful in en than in zh.

Happy to drop this and use a plain "zh" fallback if you would rather keep the
config a single string.

Translation conventions used

Two things a reviewer may notice when reading ko.json beside en.json:

• Sentences addressed to the user use the polite declarative form; buttons,
  labels and tab titles are noun phrases with no verb ending. That register
  split is the normal convention in Korean desktop UI.
• Korean particles are never attached directly after an interpolation. The
  correct particle (을/를, 이/가, 으로/로) depends on whether the
  preceding word ends in a consonant, which is not knowable at runtime for a
  skill or agent name. Strings such as Delete skill "{{name}}" permanently? are
  phrased so that a Korean noun follows the placeholder, or restructured into
  label: {{value}} form. That is why a few Korean strings do not map word for
  word onto the English clause order.

Verification

Branched from v1.29.0.

• npm run lint — clean
• npm run build — clean
• npm run release:prepare -- patch --dry-run — 1.29.0 -> 1.29.1, with ko.json
  picked up alongside the other locales
• Key parity check against en.json — 916/916 keys, 0 missing, 0 extra, 0
  placeholder mismatches, identical key order
• Ran the app with npm run tauri:dev and walked the Dashboard and the full
  Settings page in Korean, including the language picker and the About panel

Not included

No README or CHANGELOG changes, matching how feature commits are handled in
this repo — release notes are written when prepare-release runs. Glad to add a
Korean README or a changelog entry here instead if you would prefer that.