Fix pull sync rejecting every annotation: off-by-one in pos0 validation

isValidPos0 compared pos0:sub(1,7) against the 6-char string '/body/',
so it tested '/body/D' == '/body/' — false for every CRE xpointer, and
applyServerAnnotations silently dropped ALL pulled highlights/notes/
bookmarks on rolling documents. Caught by running the real plugin
code against the live server in a stubbed KOReader harness: push
stored rows on the server, pull applied nothing — matching the
reported 'neither direction syncs' symptom.

sub(1,6) now matches. Both directions re-verified end-to-end through
the actual plugin: a KOReader-style highlight pushes and lands in the
server DB (with server-derived percentage), and a metadata pull
populates a fresh device's bookmark list with web- and device-sourced
highlights (xpointer pos0s) plus web bookmarks (numeric page pos0s on
paging documents).

main.lua

@@ -1174,7 +1174,7 @@ function Bookhoard:applyServerAnnotations(annotations)
         if has_pages then
             return tonumber(pos0) ~= nil
         end
-        return pos0:sub(1, 7) == "/body/"
+        return pos0:sub(1, 6) == "/body/"
     end
 
     local function addOrUpdate(server_entry, has_text)