fix(fullnode): stable overall progress for Sapling witness rebuild

The witness-rebuild bar reset repeatedly because the daemon's "Building Witnesses
for block <h> <frac> complete" line reports per-call progress: BuildWitnessCache is
re-invoked for each connected block and each call walks from its own start height to
the tip, so the fraction restarts every time. The earlier "Setting Initial Sapling
Witness for tx <i> of <m>" counter resets per call too, so neither is a usable
overall metric.

Derive a stable, monotonic percentage from the "<n> remaining" count instead: track
the largest "remaining" seen during the phase as the full span and show how far
remaining has fallen below it. The longest pass defines 0→100%; the short per-block
follow-up passes only nudge the bar near the end rather than resetting it. The
"Setting Initial" line now only marks the phase active. Per-phase tracking resets at
phase start and every rescan-completion site.

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

src/app_network.cpp

@@ -1162,6 +1162,7 @@ void App::refreshCoreData()
                         state_.sync.building_witnesses = false;
                         state_.sync.witness_progress = 0.0f;
                         state_.sync.witness_remaining = 0;
+                        witness_rebuild_total_blocks_ = 0;
                         // Notes/witnesses were rebuilt — force a fresh history + balance pull.
                         transactions_dirty_ = true;
                         last_tx_block_height_ = -1;
@@ -2596,6 +2597,7 @@ void App::runtimeRescan(int startHeight)
             state_.sync.building_witnesses = false;
             state_.sync.witness_progress = 0.0f;
             state_.sync.witness_remaining = 0;
+            witness_rebuild_total_blocks_ = 0;
             if (ok) {
                 state_.sync.rescan_progress = 1.0f;
                 transactions_dirty_ = true;