fix: guard against font_size=0 collapsing chapter text

- Replace ?? with || when reading font_size so 0 falls back to 1.0
  (affects GET /api/settings, layout.server.ts, +layout.svelte)
- Remove the explicit 'body.fontSize !== 0' exception in PUT /api/settings
  validation so 0 is now correctly rejected as an invalid font size
- Add add_index helper + idx_chapters_idx_slug_number declaration to
  scripts/pb-init-v3.sh (idempotent UNIQUE INDEX on chapters_idx)

scripts/pb-init-v3.sh

@@ -62,6 +62,39 @@ create() {
   esac
 }
 
+# add_index COLLECTION INDEX_NAME SQL_EXPR
+# Fetches current schema, adds index if absent by name, PATCHes collection.
+add_index() {
+  COLL="$1"; INAME="$2"; ISQL="$3"
+  SCHEMA=$(curl -sf -H "Authorization: Bearer $TOK" "$PB/api/collections/$COLL" 2>/dev/null)
+  PARSED=$(echo "$SCHEMA" | python3 -c "
+import sys, json
+d = json.load(sys.stdin)
+indexes = d.get('indexes', [])
+exists = any('$INAME' in idx for idx in indexes)
+print('exists=' + str(exists))
+print('id=' + d.get('id', ''))
+if not exists:
+    indexes.append('$ISQL')
+    print('indexes=' + json.dumps(indexes))
+" 2>/dev/null)
+  if echo "$PARSED" | grep -q "^exists=True"; then
+    log "index exists (skip): $COLL.$INAME"; return
+  fi
+  COLL_ID=$(echo "$PARSED" | grep "^id=" | sed 's/^id=//')
+  [ -z "$COLL_ID" ] && { log "WARNING: cannot resolve id for $COLL"; return; }
+  NEW_INDEXES=$(echo "$PARSED" | grep "^indexes=" | sed 's/^indexes=//')
+  STATUS=$(curl -s -o /dev/null -w "%{http_code}" \
+    -X PATCH "$PB/api/collections/$COLL_ID" \
+    -H "Content-Type: application/json" \
+    -H "Authorization: Bearer $TOK" \
+    -d "{\"indexes\":${NEW_INDEXES}}")
+  case "$STATUS" in
+    200|201) log "added index: $COLL.$INAME" ;;
+    *)       log "WARNING: add_index $COLL.$INAME returned $STATUS" ;;
+  esac
+}
+
 # add_field COLLECTION FIELD_NAME FIELD_TYPE
 # Fetches current schema, appends field if absent, PATCHes collection.
 # Requires python3 for safe JSON manipulation.
@@ -294,4 +327,8 @@ add_field "app_users"      "polar_subscription_id"  "text"
 add_field "user_library"   "shelf"                  "text"
 add_field "user_sessions"  "device_fingerprint"     "text"
 
+# ── 6. Indexes ────────────────────────────────────────────────────────────────
+add_index "chapters_idx" "idx_chapters_idx_slug_number" \
+  "CREATE UNIQUE INDEX idx_chapters_idx_slug_number ON chapters_idx (slug, number)"
+
 log "done"