fix(sync): bootstrap book UUID via push before first pull

On the first open of a freshly downloaded book there is no cached
bookhoard UUID yet, since it is only learned from a successful push
response. Pulling at that point simply failed with "Push progress
first", which also broke auto sync on open.

- onReaderReady auto sync: if no UUID is cached, push once to
  bootstrap identity (the server resolves the book by format-aware
  SHA-256 and returns the UUID, which we then cache); otherwise pull
  as before
- getProgress: when no UUID is cached, instead of showing the dead-end
  "Push progress first" message, show "Linking this book to Bookhoard
  first…", push once via _bootstrapUUIDThenPull, and retry the pull
  once the UUID is cached
- _bootstrapUUIDThenPull: new helper that pushes, waits 2s for the
  UUID to be cached, then performs the originally-requested pull,
  falling back to an error message on failure

After this bootstrap both push and pull work without ordering.

main.lua

@@ -100,7 +100,17 @@ end
 function Bookhoard:onReaderReady()
     if self.settings.auto_sync then
         UIManager:nextTick(function()
-            self:getProgress(true, false)
+            -- On the first open of a freshly downloaded book there is no cached
+            -- bookhoard UUID yet (it is only learned from a successful push
+            -- response). Pulling now would just fail with "Push progress first".
+            -- Instead, push once to bootstrap identity: the server resolves the
+            -- book by SHA-256 (format-aware) and returns the UUID, which we then
+            -- cache. After that, push and pull both work without ordering.
+            if not self:getBookhoardUUID() then
+                self:updateProgress(true, false)
+            else
+                self:getProgress(true, false)
+            end
         end)
     end
     self:registerEvents()
@@ -992,12 +1002,16 @@ function Bookhoard:getProgress(ensure_networking, interactive)
 
     local book_uuid = self:getBookhoardUUID()
     if not book_uuid then
+        -- No cached UUID yet (first open of a freshly downloaded book). Bootstrap
+        -- it by pushing once: the server resolves the book by SHA-256 and returns
+        -- the UUID, which the push handler caches. Then retry the pull.
         if interactive then
             UIManager:show(InfoMessage:new{
-                text = _("No Bookhoard UUID for this document. Push progress first."),
+                text = _("Linking this book to Bookhoard first…"),
                 timeout = 3,
             })
         end
+        self:_bootstrapUUIDThenPull(interactive)
         return
     end
 
@@ -1008,6 +1022,23 @@ function Bookhoard:getProgress(ensure_networking, interactive)
     self.pull_timestamp = now
 end
 
+-- Establish the bookhoard UUID for the current document by pushing once, then
+-- (once cached) perform the originally-requested pull. Used when a pull is
+-- requested before any push has run on a newly downloaded book.
+function Bookhoard:_bootstrapUUIDThenPull(interactive)
+    self:updateProgress(false, false)
+    UIManager:scheduleIn(2, function()
+        if self:getBookhoardUUID() then
+            self:_doGetProgress(interactive)
+        elseif interactive then
+            UIManager:show(InfoMessage:new{
+                text = _("Could not link this book. Check your network and try again."),
+                timeout = 3,
+            })
+        end
+    end)
+end
+
 function Bookhoard:_doGetProgress(interactive)
     local book_uuid = self:getBookhoardUUID()
     if not book_uuid then return end