feat: phase 1 of AI pipeline hardening — single LLM client + tier-aware models

Implements phase 1 of AI_PIPELINE_HARDENING_PLAN.md. Every Anthropic call
now goes through one module that owns retry, timeout, abort, structured-
output parsing, schema validation, and best-effort call telemetry.

* src/lib/llm.js — single callLLM entry point. Resolves model per tier
  (fast / standard / reasoning) with admin:model legacy fallback for the
  standard tier; 60s default timeout via AbortController; balanced-brace
  JSON extraction; LLMHttpError, LLMTruncatedError, LLMOutputError, and
  LLMValidationError surface clearly distinct failure modes.
* src/lib/llmRetry.js — exponential backoff with full jitter, retries
  only on transient HTTP statuses, honours Retry-After up to 60s, never
  retries on AbortError.
* src/lib/llmSchemas.js — Zod schemas for every structured task plus
  normalizeHandbookResult (collapses legacy "executes" relations into
  the canonical "executed_by" vocabulary).
* src/lib/api.js — thin shim over callLLM so existing callers (extraction
  pipeline, learning, quiz, R42, knowledge graph) keep working unchanged.
* src/lib/__tests__/ — 32 Vitest cases covering parse paths, error
  surfaces, simulation mode, model resolution, and schema validation.
* src/pages/Admin/index.jsx — three model inputs (fast / standard /
  reasoning) replacing the single legacy field; legacy value falls back
  for the standard tier so existing overrides survive.

Adds Zod and Vitest, plus an "npm run test" script.

Also cleans up the pre-existing repo-wide ESLint failures so phase 1's
"npm run lint passes" acceptance criterion can be checked: drops unused
React imports across the JSX tree (React 19 JSX runtime auto-imports),
attaches cause to rethrown errors in the service modules, ignores
pb_migrations in the ESLint config (PocketBase JSVM globals), and
removes one dead handleCreateCustom function in Leren.jsx. A real
behaviour bug surfaced in Testen.jsx — the quiz timer captured a stale
finishQuiz via setInterval closure; now updated via finishQuizRef so the
timer always invokes the latest callback.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/Leren.jsx

@@ -1,14 +1,13 @@
-import React, { useState, useEffect } from 'react';
-import { BookOpen, CheckCircle, Loader, ArrowRight, Plus, Search, ChevronLeft, MessageSquare, Calendar, TrendingUp } from 'lucide-react';
+import { useState, useEffect } from 'react';
+import { BookOpen, CheckCircle, Loader, ArrowRight, ChevronLeft, MessageSquare, Calendar, TrendingUp } from 'lucide-react';
 import { motion, AnimatePresence } from 'framer-motion';
 import { Link } from 'react-router-dom';
 import Card from '../components/ui/Card';
 import Button from '../components/ui/Button';
 import Tag from '../components/ui/Tag';
-import Input from '../components/ui/Input';
 import LearningContentViewer from '../components/ui/LearningContentViewer';
 import { useApp } from '../store/AppContext';
-import { getAssignedTopic, generateLearningContent, getCachedContent, generateCustomTopic } from '../lib/learningService';
+import { getAssignedTopic, generateLearningContent, getCachedContent } from '../lib/learningService';
 import { getUpcomingWeeks, getQuarterProgress, getYearProgress, getQuarterName, getQuarterForWeek, hasCurriculum as checkHasCurriculum } from '../lib/curriculumService';
 import * as db from '../lib/db';
 
@@ -27,7 +26,7 @@ const Leren = () => {
   const [error, setError] = useState(null);
   
   // Custom Topic
-  const [customTopicQuery, setCustomTopicQuery] = useState('');
+  const [customTopicQuery] = useState('');
   
   // Weekly status
   const [weeklyDone, setWeeklyDone] = useState(false);
@@ -107,26 +106,6 @@ const Leren = () => {
     }
   };
 
-  const handleCreateCustom = async (e) => {
-    e.preventDefault();
-    if (!customTopicQuery.trim()) return;
-    
-    setIsLoading(true);
-    setView('creating');
-    setError(null);
-    try {
-      const newTopic = await generateCustomTopic(customTopicQuery);
-      setAllTopics(await db.getTopics());
-      setCustomTopicQuery('');
-      handleOpenTopic(newTopic);
-    } catch (e) {
-      setError(e.message);
-      setView('overview');
-    } finally {
-      setIsLoading(false);
-    }
-  };
-
   const doComplete = async () => {
     setSessionDone(true);
     if (!weeklyDone) {