Replace string concatenation (greeting + displayName) with two
i18next keys: greetingWithName (with {{name}} interpolation) and
greetingDefault (standalone, no name).

This lets each locale choose natural phrasing independently:
- zh-CN: "嗨，同学" / "嗨，Alice"
- en-US: "Hi there" / "Hi, Alice"
- Future locales can avoid gender issues by choosing genderless defaults

Also widen the t() type signature to accept interpolation options.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Replace hardcoded zh-CN/en-US imports with i18next-resources-to-backend
and dynamic import(`./locales/${language}.json`). Bundler scans the
locales/ directory at build time, so adding a new language now requires
only dropping a JSON file — zero changes to existing code.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

LanguageDetector ran during i18next init(), detecting browser language
before React hydrated — server rendered zh-CN while client switched to
en-US immediately, causing a hydration mismatch.

Fix: remove i18next-browser-languagedetector; init with a fixed lng
(zh-CN) so server and client agree on the first render. Language
detection is now done in I18nProvider's useEffect after hydration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Replace manual locale validation and startsWith('zh') prefix matching
with i18next's built-in fallback mechanism. Now changeLanguage() is
called with navigator.language directly — if the exact locale has no
JSON file, i18next automatically falls back to fallbackLng.

Also widen Locale type from union to string so adding new languages
doesn't require modifying types.ts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Add lib/i18n/course-languages.ts with curated language list
  (zh-CN, zh-TW, en-US, ja, ko, fr, de, es, pt, ru, ar) including
  native labels and English prompt names
- Course language defaults to UI locale on first visit; once user
  explicitly picks a language, that choice persists across sessions
- Replace toggle button with dropdown selector showing native labels
- Widen language types from 'zh-CN'|'en-US' to string throughout
- Fix hardcoded language ternaries in LLM prompt injection:
  - prompt-builder.ts: use getCourseLanguagePromptName()
  - classroom-generation.ts: remove normalizeLanguage() that forced
    all non-English to zh-CN
  - PBL system prompt, agent templates, generate-pbl: append language
    instruction for non-zh/en languages
  - quiz-grade API: add language suffix for grading feedback

Closes #327

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

quiz-view was passing the UI locale to the grading API, causing AI
feedback to follow the student's answer language instead of the course
language. Now reads stage.language from the store.

Also strengthen the grading prompt: explicitly instruct the LLM to
write comments in the course language regardless of the student's
input language.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Reverts f7bd6bd and 5bb5133 — these are independent features that
should go into a separate branch/PR.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

These files are replaced by lib/i18n/locales/*.json and are no longer
imported anywhere in the codebase.

Removed: chat.ts, common.ts, generation.ts, settings.ts, stage.ts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Resolve modify/delete conflict on lib/i18n/generation.ts — this file was
removed in the i18next branch (translations migrated to JSON locale files)
and modified on main. All new keys from main are already present in the
JSON locales, so the TS file is no longer needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add classroom.rename, classroom.renamePlaceholder, and
classroom.renameFailed that were added to generation.ts on main
but missing from the JSON locale files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…cale resolution

- Extract shared <LanguageSwitcher /> component from page.tsx and header.tsx
- Derive Locale type from supportedLocales registry (as const satisfies)
- Add resolveLocale() to match browser language prefixes (e.g. 'en' → 'en-US')
- Remove config.ts changes (nonExplicitSupportedLngs + inline resources) that
  broke translation loading when combined with resourcesToBackend

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Remove greetingDefault key; greeting always uses greetingWithName
  with displayName (falls back to profile.defaultNickname)
- Change en-US defaultNickname from "Student" to "Learner"
- Add TRANSLATION_GUIDE.md documenting how to add languages and
  explaining keys with non-obvious UX design intent

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>