fix(vocational): send taskEngineMode on the on-demand scene generation path

[ly-wang19]

Fixes #715.

Problem

The vocational task-engine gate is driven by requirements.taskEngineMode in the /api/generate/scene-content request body (resolveVocationalActive → applyOutlineFallbacks({ allowProceduralSkill })). Only the first scene sends it (app/generation-preview/page.tsx). Scenes 2..N and retries are generated by useSceneGenerator.generateRemaining / fetchSceneContent, which never included requirements — so vocationalActive was false for them and applyOutlineFallbacks rewrote every procedural-skill outline to diagram. The outline says procedural-skill, the content path strips it; most scenes of a vocational course silently lose the task-engine widget. (Full trace in #715.)

Fix (minimal, client-side)

taskEngineMode is already persisted on the stage (lib/types/stage.ts, lib/utils/database.ts, written in stage-storage.ts), so this mirrors what scene 1 already does:

• add taskEngineMode?: boolean to GenerationParams
• populate it at the single generateRemaining(...) call site from stage.taskEngineMode
• include requirements: { taskEngineMode: params.taskEngineMode === true } in both fetchSceneContent bodies

The retry path (retrySingleOutline) inherits it automatically — it reuses lastParamsRef.current. Server contract is unchanged; the flag is still ANDed with the OPENMAIC_ENABLE_VOCATIONAL env gate, so this is not a security/routing change — it only stops the client from dropping a flag it already sends for scene 1.

Note on approach

This is the smallest fix that restores parity with the first-scene path. If you'd prefer a server-authoritative shape instead — e.g. persisting taskEngineMode onto the outline so the content route doesn't depend on the client re-sending it — I'm happy to revise this PR that way; it's your feature and I'd rather match your intended design.

Verification

• prettier clean, eslint clean, tsc --noEmit error count unchanged (only pre-existing unresolved @maic/* workspace-module errors; neither edited file appears).
• No test added: the server gate this feeds is already covered by tests/generation/scene-content-route-vocational-gate.test.ts; the gap is purely the client hook's request body, and the repo has no React-hook (renderHook + fetch-mock + Zustand) harness to assert it without adding disproportionate test infra. Happy to add one if you'd like to introduce that harness.