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/util/i18n.cpp

@@ -817,6 +817,8 @@ void I18n::loadBuiltinEnglish()
         strings_["sb_syncing_basic"] = "Syncing %.1f%% (%d left)";
         strings_["sb_rescanning_pct"] = "Rescanning %.0f%%";
         strings_["sb_rescanning"] = "Rescanning";
+        strings_["sb_building_witnesses_pct"] = "Rebuilding witnesses %.0f%%";
+        strings_["sb_building_witnesses"] = "Rebuilding witnesses";
         strings_["sb_importing_keys"] = "Importing keys";
         strings_["sb_daemon_not_found"] = "Daemon not found";
         strings_["sb_loading_config"] = "Loading configuration...";