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Korean speech level for Coucou Flow

Why

Korean has grammatical speech levels, and the right one depends on who you are writing to. People often talk in a casual register but write to colleagues, customers and teachers in a polite one, and a mail in 반말, or a chat message to a superior at the wrong level, reads as rude. Dictation therefore often needs a rewrite after the fact. Flow's clean-up already fixes punctuation and fillers; this lets Claude also fix the level.

What

• A "Korean speech level" setting: as spoken (default), 해요체 (polite), 합니다체 (formal), 해체 (casual).
• Applied wherever Claude already rewrites a dictation: clean-up, prompt mode and translation.
• The rule asks for the sentence a native speaker would say at that level, not only a new verb ending, and keeps the sentence type (a suggestion stays a suggestion). Example: 너 뭐 좋아해? → 어떤 걸 좋아하세요? (해요체).
• The row sits on the Styles page, under the clean-up and the default style, and is shown only on a Korean Mac, when Korean is the translation language, or once a level has been chosen. It is off by default: nothing changes for anyone else.
• No new dependency and no new network call (same Claude request as before, with one more instruction).

Notes

• New strings are translated in en, fr, ko and zh-Hant (the check requires zh-Hant); other languages fall back to English.
• Possible follow-up: the same idea for other languages that have speech levels or a formal/informal "you" (Japanese, German, French, Spanish, Russian…), ideally per app through the app styles.
• Related: feat(i18n): add Korean localization across macOS, Windows and Linux #880 (Korean localization). This PR does not depend on it.

Test plan

• scripts/test-korean-level.sh (added to CI), scripts/test-flow-text.sh, scripts/test-localization.py
• Build with no new warnings
• Dictated casual Korean with 해요체 on a Korean Mac: ends polite, the pronoun and wording follow
• 합니다체 checked by dictation (오늘 회의 몇 시야 → 오늘 회의는 몇 시입니까?)
• 해체 checked by dictation

Screenshots

The setting, on the Styles page (under the clean-up and the default style):

A dictation rewritten in 합니다체 (see also #952 for the history):