feat: 为故事转剧本流程添加语言选择器

[Frty47222]

Summary

• What changed:
• Why:

Architecture Plan Sync (Required)

• Master plan file: /Users/earth/Desktop/waoowaoo/docs/architecture-unification-master-plan.md
• I updated the phase status board (✅/🔄/⏸/⚠️) in the same PR.
• I recorded concrete file-level changes under the corresponding phase.
• I kept the plan date/current-context consistent with the code changes in this PR.

Phase Status Delta (Required)

Phase	Before	After	Evidence (file path)
e.g. Phase 1.2	🔄	✅	src/lib/query/task-target-overlay.ts

Guardrails Checklist (Required)

• No compatibility layer introduced.
• No silent fallback introduced.
• No hidden default or fake data introduced.
• Errors remain explicit and traceable.

Validation

• Commands run:
• Manual verification:

Risks

• Known impact/risk:
• Follow-up tasks: