feat(create-lesson): multi-select book sections + exclusion heuristic + batch generation (#1949)

[astrapi69]

Description

Turns the #1927 book-file section picker from a single-select dropdown into a checkbox multi-select, and generates one standalone lesson per selected section in a single batch run (one set, N lessons). Builds on #1927 (Phase 2a upload + single-select picker).

Binding design decision: batch produces N standalone lessons (one per section), not one giant lesson — short, focused micro-learning units mapped 1:1 to the book's author-set chapter boundaries. The "one run -> one lesson" model is preserved structurally, extended to "one batch run -> N lessons".

What changed

• Exclusion heuristic (section-heuristics.ts): multilingual (DE/EN/FR) predicate that deselects likely front/back matter (preface, glossary, table of contents, imprint, index, …) by default while keeping every section visible and manually checkable.
• Batch generation (generate-book-lessons.ts): extracts the single chunk→lesson step shared by the single and batch paths (no second generation logic) plus a fault-tolerant batch loop — a per-section failure is recorded and the run continues — with progress reporting and an oversized-section guard.
• Lesson assembly (book-to-lesson.ts): buildBookLessons / buildBookLessonsUserSetInput assemble N lessons with unique ids into one set; the single builders delegate to them. Regression-preserving: the single paste path passes an empty entry title so the lesson id/title still track meta.title.
• UI (BookFileUpload / BookTextStep / BookSteps / CreateLesson): checkbox list; exactly one selected keeps the existing "Insert into text field" single path (with the replace-confirm dialog and preview), more than one shows "Generate N lessons"; document-order emission (not selection order); batch progress ("Generating lesson 2 of 5…") + summary (X of N succeeded, which failed); review lists N lessons; save persists the whole set.

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Frontend Vitest passes locally (full suite: 733 files / 7643 tests green; type-check clean; ESLint --max-warnings 0 clean on the changed files)
• TDD RED-first: new tests cover the heuristic (DE/EN/FR titles + default selection), the batch loop (order, per-section failure isolation, progress, oversized guard), multi-lesson set assembly (unique ids), the multi-select picker (default deselection + still-checkable, single-path regression, document-order batch emit), and a CreateLesson integration test that uploads a 3-section markdown file (incl. a Vorwort), batch-generates, and saves one set with two lessons.
• Device verification (browser) still pending — see checklist below.

Documentation

• Manual test plan updated (DE + EN) with the multi-select / heuristic / batch flow (TESTPLAN-PFLICHT)
• German translations updated — new i18n keys added to all 11 catalogs and mirrored to the frontend JSON (make sync-i18n); verify-i18n-scripts clean

Checklist

• I have performed a self-review of my own code
• Public functions carry docstrings/TSDoc; behaviour is covered by tests
• My changes generate no new warnings or errors

Screenshots / Logs

Manual device verification (upload with front matter, multi-select batch, single-select regression) is the remaining human step per the Visual-Device-Check; automated coverage is green.

Refs #1927
Closes #1949

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