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/app.h

@@ -306,6 +306,7 @@ public:
     bool isUsingEmbeddedDaemon() const { return supportsEmbeddedDaemon() && use_embedded_daemon_; }
     void setUseEmbeddedDaemon(bool use) { use_embedded_daemon_ = use && supportsEmbeddedDaemon(); }
     void rescanBlockchain();  // restart daemon with -rescan flag
+    void repairWallet();  // restart daemon with -zapwallettxes=2 (wipe & rebuild wallet tx records)
     void deleteBlockchainData();  // stop daemon, delete chain data, restart fresh
     bool stopDaemonForBootstrap(); // stop daemon + disconnect for bootstrap, returns true if was running
     bool isBootstrapDownloading() const { return bootstrap_downloading_; }