fix(lite): honor the backend sync result with retry and a failed state

A failed backend sync was silently swallowed: the reply was never
inspected, so a sync that errored out left the wallet showing a
"syncing" state that never completed and never reported a problem.

- The sync and rescan replies are now parsed. On failure the partial
  progress is saved and the sync is retried with exponential back-off
  (5s doubling up to 60s, six attempts in all), then an explicit failed
  state is published: done, sync not complete, and the backend's error
  text carried through.
- A failed rescan is retried as a plain sync, because a rescan has
  already cleared the wallet and re-running it would throw away the
  partial progress just saved.
- A terminal error, such as a reorg deeper than the wallet can unwind,
  stops the retries immediately instead of waiting out the attempts.
- Recovery is observed, not guessed: once a later refresh's server-bound
  reads all succeed, the failed state clears itself.

Behaviour change: a sync failure now surfaces to the user as a failed
state with a message, instead of a frozen "syncing", and the client
retries on its own with back-off.

Tests: a parser case for the success, failure, and terminal replies, and
fake-backend controller cases for retrying after a failure, giving up
after the attempt cap, a terminal reorg stopping at once, a failed
rescan retrying as sync, cancel stopping the back-off, and the failed
state self-healing on a good refresh.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/data/wallet_state.h

@@ -147,6 +147,7 @@ struct SyncInfo {
     double verification_progress = 0.0;
     bool syncing = false;
     std::string best_blockhash;
+    std::string lite_error;   // lite: last sync failure text, "" = none
     
     // Rescan state (detected from daemon output)
     bool rescanning = false;