Add chapter backward/forward navigation in read_book UI

- Each chapter card now has a footer with Prev/Next chapter anchor links
- First chapter shows only Next; last chapter shows 'End of Book'
- Back to Top link on every chapter footer
- Added get_chapter_neighbours() helper in story/bible_tracker.py for
  programmatic chapter sequence navigation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/read_book.html

@@ -48,6 +48,27 @@
                 </div>
             </div>
 
+            <!-- Chapter Navigation Footer -->
+            <div class="card-footer bg-transparent d-flex justify-content-between align-items-center py-2">
+                {% if not loop.first %}
+                    {% set prev_ch = manuscript[loop.index0 - 1] %}
+                    <a href="#ch-{{ prev_ch.num }}" class="btn btn-sm btn-outline-secondary">
+                        <i class="fas fa-arrow-up me-1"></i>Ch {{ prev_ch.num }}
+                    </a>
+                {% else %}
+                    <span></span>
+                {% endif %}
+                <a href="#" class="btn btn-sm btn-link text-muted small py-0">Back to Top</a>
+                {% if not loop.last %}
+                    {% set next_ch = manuscript[loop.index0 + 1] %}
+                    <a href="#ch-{{ next_ch.num }}" class="btn btn-sm btn-outline-secondary">
+                        Ch {{ next_ch.num }}<i class="fas fa-arrow-down ms-1"></i>
+                    </a>
+                {% else %}
+                    <span class="text-muted small fst-italic">End of Book</span>
+                {% endif %}
+            </div>
+
             <!-- Rewrite Modal -->
             <div class="modal fade" id="rewriteModal{{ ch.num|string|replace(' ', '') }}" tabindex="-1">
                 <div class="modal-dialog">