Fix/qdn publishing check

[nbenaglia]

#30:

• Old-format migration now writes lastUpdated: 0 instead of Date.now(). Any valid QDN timestamp (> 0) will always be considered newer, so the startup sync fetches from QDN instead of publishing spuriously.

• Before publishing when QDN returns null, the code now reads a q-wallets-addressbook-published-${coinType} sentinel key from localStorage. If the hash of local entries matches the last successfully published hash, the publish (and its permission dialog) is skipped. This prevents repeated proposals during QDN propagation delay.
  publishToQDN now captures lastUpdated = Date.now() before the async operations and returns it (or null on failure). Both call-sites in the startup sync use the returned value to write localStorage, so local and QDN timestamps are identical after a publish — the next startup goes straight to the equal-timestamp path with no further hash comparison or dialog.

• publishToQDN also writes the published hash to the sentinel key on every successful publish (startup or debounced), keeping the propagation-delay guard in sync.