feat(fullnode): show Sapling note witness-rebuild progress

The daemon's post-rescan witness rebuild ("Building Witnesses for block ...")
is a distinct, often-long phase that previously showed only as an indeterminate
"Rescanning..." with no progress. Parse the daemon's witness-build log lines and
surface a dedicated progress indicator.

- Parse "Building Witnesses for block <h> <frac> complete, <n> remaining" (and the
  earlier "Setting Initial Sapling Witness for tx ..., <i> of <m>") from daemon
  output, extracting a 0..1 fraction and remaining-block count.
- New SyncInfo fields building_witnesses / witness_progress / witness_remaining,
  cleared at every rescan-completion site (warmup-end, getrescaninfo poll, runtime
  rescan callback, daemon-log "finished").
- Status bar shows "Rebuilding witnesses NN%" (priority over the generic rescan
  text); the loading overlay (shown during -rescan warmup) gets a labelled witness
  progress bar with the remaining-block count.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app_network.cpp

@@ -1159,6 +1159,9 @@ void App::refreshCoreData()
                         rescan_confirmed_active_ = false;
                         state_.sync.rescan_progress = 1.0f;
                         state_.sync.rescan_status.clear();
+                        state_.sync.building_witnesses = false;
+                        state_.sync.witness_progress = 0.0f;
+                        state_.sync.witness_remaining = 0;
                         // Notes/witnesses were rebuilt — force a fresh history + balance pull.
                         transactions_dirty_ = true;
                         last_tx_block_height_ = -1;
@@ -2590,6 +2593,9 @@ void App::runtimeRescan(int startHeight)
             rescan_confirmed_active_ = false;
             state_.sync.rescanning = false;
             state_.sync.rescan_status.clear();
+            state_.sync.building_witnesses = false;
+            state_.sync.witness_progress = 0.0f;
+            state_.sync.witness_remaining = 0;
             if (ok) {
                 state_.sync.rescan_progress = 1.0f;
                 transactions_dirty_ = true;