Phase 6: accordion in /learn; README feature notes; tests green

- collapse chapters/sections via styled <details> with chevrons
- identity testcontainers smoke passes on JDK 21
- README: student UX, comments, JDK note (logback ThrowableProxy in
  fat-jars → restart recovery), data dirs

web/src/main/resources/static/css/app.css

@@ -231,3 +231,16 @@ code.code { padding: 0.125rem 0.5rem; border-radius: 0.25rem; font-family: monos
 .chip-neg { background: #b3283b; color: #fff; }
 
 /* Defense timer */
+/* Accordion (главы/разделы в /learn) */
+details.accordion > summary.acc-head { list-style: none; cursor: pointer; display: flex; align-items: center; justify-content: space-between; }
+details.accordion > summary::-webkit-details-marker { display: none; }
+details.accordion[open] > .acc-head, details.accordion > summary { margin-bottom: 0; }
+.acc-arrow { width: 10px; height: 10px; border-right: 2px solid var(--c-text-muted); border-bottom: 2px solid var(--c-text-muted); transform: rotate(45deg); transition: transform 0.2s; margin-left: .75rem;}
+details.accordion[open] > summary > .acc-arrow { transform: rotate(-135deg); }
+.acc-title { margin: 0; font-size: 1.0625rem; }
+.acc-body { border-top: 1px solid var(--c-border); }
+details.sub-acc > summary.sub-head { list-style: none; cursor: pointer; }
+details.sub-acc > summary::-webkit-details-marker { display: none; }
+.sub-arrow { display: inline-block; width: 8px; height: 8px; border-right: 2px solid var(--c-text-muted); border-bottom: 2px solid var(--c-text-muted); transform: rotate(45deg); transition: transform 0.2s; margin-right: 6px; vertical-align: 2px;}
+details.sub-acc[open] > summary .sub-arrow { transform: rotate(-135deg); }
+.sub-actions { margin: 0 0 2px 18px; }