Feature: parent-managed kids EPUB reading mode

[evan188199-tech]

Problem

The immersive reading workspace targets independent readers. Young learners need a simpler EPUB experience with age-appropriate presentation, controlled navigation, vocabulary practice, read-aloud support, and parent management.

This depends on the immersive reading foundation in #719. It should be built as focused follow-up PRs rather than one large feature PR.

Proposal

Add a parent-managed kids reading mode:

• Child profiles with display name, avatar, birth date or derived age band, reading level, and language preference.
• Parent-managed library, profile lifecycle, PIN/exit protection, and review of reading activity.
• Standalone child entry that does not expose the full workspace navigation.
• Illustrated EPUB reader with simplified controls, text sizing, read-aloud, and translation support.
• Age-tiered vocabulary quiz with deterministic fallback when model generation is unavailable.
• Clear boundaries for TTS and translation failures so reading remains usable offline or with partial services.

Suggested PR sequence:

1. Child profile and parent authorization data models/API.
2. Parent library management and child profile management.
3. Child reader entry and navigation protection.
4. Kids EPUB presentation, TTS, and translation integration.
5. Age-tiered vocabulary quiz and learning review.

Privacy and safety requirements

• Child profiles must not be exposed to unrelated accounts.
• Parent actions require the authenticated parent/adult context; child pages remain exempt but narrowly scoped.
• PIN protection must prevent casual exit from the child session without obscuring recovery for the parent.
• Collect the minimum profile data needed; do not infer sensitive attributes beyond the selected age/level.
• Third-party model, TTS, and translation failures must degrade to readable content and a clear retry state.

Acceptance criteria

• A parent can create, edit, archive, and delete child profiles and manage their library.
• A child can open only their assigned books and cannot reach general workspace routes through visible navigation.
• Reader controls work on desktop and mobile layouts without overlapping text or controls.
• TTS, translation, and quiz errors expose actionable retry behavior and never block continued reading.
• Vocabulary quizzes vary by age band and have a deterministic fallback.
• API and UI tests cover authorization boundaries, profile isolation, empty/error/loading states, and responsive reader layout.

[evan188199-tech]

Implementation notes from a local prototype that may help scope this feature:\n\n- Keep child-facing reading routes focused and exempt from adult session requirements.\n- Derive the quiz tier from birth date with safe fallback bands (3-5, 6-8, 9-12).\n- Gate parent profile/settings management behind a PIN or adult login; do not require that gate on the child reading flow.\n- Provide deterministic vocabulary/translation fallback when generated quiz content is unavailable.\n- Cover the separation between child routes and management routes in backend and frontend tests.\n\nHappy to split the prototype into reviewable PRs once the desired integration point is agreed.