feat: add Russian support for generation and PBL runtime flows

[GRIZZZZZLY]

Summary

This PR is rebased onto the current main and repurposed as a follow-up to the already merged ru-RU locale work.

Instead of adding locale wiring again, it focuses only on the remaining Russian runtime, generation, grading, and PBL gaps.

Related Issues

Closes #402.

Changes

• enable ru-RU as a generation language in the generation flow UI
• carry ru-RU through server-side generation normalization and types
• add Russian support to quiz grading prompts and fallback messages
• add Russian fallback strings in scene outline generation helpers
• improve Russian prompt formatting for generation-related image/context text
• add Russian language mapping in orchestration prompt building
• add Russian support across PBL generation prompts, system prompts, and agent templates

Why this matters

ru-RU locale support is already available in main, but runtime behavior was still incomplete in several places and often fell back to English or Chinese.

This PR closes those runtime gaps so Russian works more consistently across:

• generation
• grading
• outline helpers
• PBL flows

Verification

• pnpm lint on changed files ✅
• pnpm build ✅
• pnpm test ✅

AI Assistance

This PR was prepared with AI assistance, then reviewed and validated locally before push.

Notes

This PR intentionally does not touch locale JSON wiring. It is a follow-up focused only on runtime/generation/PBL Russian support on top of the current main.

[GRIZZZZZLY]

[GRIZZZZZLY]

Hi! I fixed the formatting issues in this PR and pushed an update.\n\nThe latest workflow run now shows action_required rather than a code failure, so it looks like GitHub Actions may need maintainer approval to run for this fork PR.\n\nCould someone please approve and run the workflows for this PR? Thanks!

[wyuc]

Hey, thanks for putting this together!

Since this PR was opened, we've refactored the i18n system. Adding a new locale now only needs two changes:

1. A lib/i18n/locales/<locale>.json file with all translation keys
2. One line in lib/i18n/locales.ts to register it

See the Translation Guide and #365 (Japanese) for reference.

Could you rebase onto current main and simplify your PR to follow this pattern? The translation strings you've already done are great, it's mainly the wiring that needs updating. Happy to merge once it's aligned.

[wyuc]

Hey, thanks for putting this together!

Since this PR was opened, we've refactored the i18n system. Adding a new locale now only needs two changes:

1. A lib/i18n/locales/<locale>.json file with all translation keys
2. One line in lib/i18n/locales.ts to register it

See the Translation Guide and #365 (Japanese) for reference.

Could you rebase onto current main and simplify your PR to follow this pattern? The translation strings you've already done are great, it's mainly the wiring that needs updating. Happy to merge once it's aligned.

[GRIZZZZZLY]

@wyuc rebased onto current main and narrowed this PR to the remaining Russian runtime/generation/PBL support on top of the already merged ru-RU locale work. CI is now green. Would appreciate a review when you have a moment