feat(fullnode): add "Repair Wallet" (-zapwallettxes=2) to Settings

When a note's stored record is corrupt or its tx isn't in the canonical chain,
z_sendmany fails to build a valid sapling spend proof even after a full -rescan,
because a plain rescan replays witnesses but keeps the existing tx/note records.
The zcashd repair for this is -zapwallettxes=2, which deletes all wallet tx/note
records and rebuilds them from the chain (keys/addresses preserved).

Adds a RepairWallet lifecycle operation that mirrors the existing -rescan plumbing
(one-shot zapOnNextStart flag on the embedded daemon; -zapwallettxes=2 implies and
supersedes -rescan), an App::repairWallet() that reuses the rescan status UI (so the
status bar + warmup-end completion detection apply), and a confirmed "Repair Wallet"
button + dialog in Settings → node maintenance (embedded daemon only).

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

src/daemon/embedded_daemon.cpp

@@ -482,8 +482,14 @@ bool EmbeddedDaemon::start(const std::string& binary_path)
         args.push_back("-maxconnections=" + std::to_string(max_connections_));
     }
 
-    // Add -rescan flag if requested (one-shot)
-    if (rescan_on_next_start_.exchange(false)) {
+    // Add wallet-repair flag if requested (one-shot). -zapwallettxes=2 wipes all wallet tx/note
+    // records and rebuilds them from the chain; it implies -rescan, so don't also pass -rescan.
+    if (zap_on_next_start_.exchange(false)) {
+        DEBUG_LOGF("[INFO] Adding -zapwallettxes=2 flag for wallet repair (zap & rebuild)\n");
+        args.push_back("-zapwallettxes=2");
+        rescan_on_next_start_.store(false);  // implied by zap; avoid redundant -rescan
+    } else if (rescan_on_next_start_.exchange(false)) {
+        // Add -rescan flag if requested (one-shot)
         DEBUG_LOGF("[INFO] Adding -rescan flag for blockchain rescan\n");
         args.push_back("-rescan");
     }