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.h

@@ -635,6 +635,11 @@ private:
     // Set when a bootstrap completes; consumed once the daemon is connected to auto-run a rescan
     // that reconciles the preserved wallet.dat against the freshly-imported chain.
     bool post_bootstrap_rescan_pending_ = false;
+    // Largest "blocks remaining" seen during the current witness-rebuild phase. The daemon's
+    // "Building Witnesses for block" fraction resets every call (it's re-invoked per connected
+    // block, each walking from its own start height to the tip), so we derive a stable, monotonic
+    // overall percentage from how far "remaining" has fallen below this peak. Reset per phase.
+    int witness_rebuild_total_blocks_ = 0;
     bool opid_poll_in_progress_ = false;
     // Consecutive Core-refresh cycles where BOTH core RPCs failed → likely a dead
     // connection. After kCoreFailuresBeforeDisconnect, tear down and reconnect.