feat(node): detect an unreadable block DB on startup and offer a one-click reindex

When a daemon update changes the block-index on-disk format (or the index is
corrupt), dragonxd aborts at startup — "non-canonical optional discriminant" →
"Error loading block database. Aborted." — and the wallet silently shows a zero
balance. Previously the connect loop just crash-restarted into the same abort up
to 3x and then reported a bare "Daemon crashed N times", with no path forward.

Now:
- daemon/daemon_startup_diagnosis.h: pure blockDbOutputLooksBroken() classifies
  the crashed node's captured console output (the fatal block-DB markers).
- The connect loop detects it on the FIRST abort, STOPS crash-restarting into the
  same failure (each retry reloads the whole index — wasteful), and offers a fix.
- A one-shot -reindex flag (EmbeddedDaemon::setReindexOnNextStart → DaemonController
  forwarder → args) rebuilds the block index + chainstate from the intact raw
  blocks; App::reindexBlockDatabase() arms it and un-gates the loop to restart.
- An auto-shown dialog (renderBlockDbReindexDialog) + a notification explain the
  situation ("your coins are safe; the node just can't load the chain") and offer
  a one-click "Rebuild block database". Full-node only (gated), lite-safe.

This is the exact trap behind a real "big wallet shows no funds" report: a
post-format-change daemon over pre-change chaindata. Reindex also fixes a plain
corrupt index.

Adds testBlockDbOutputDiagnosis (the abort sequence + individual markers trip it;
normal startup / wallet-corruption / asmap errors do not). Suite green (1/1).

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

src/app.h

@@ -895,6 +895,11 @@ private:
     // sets these and defers to renderSwitchStopDaemonDialog; confirming re-calls switchToWallet(w, true).
     bool show_switch_stop_daemon_confirm_ = false;
     std::string pending_switch_wallet_file_;
+    // Block-database recovery: set when the embedded node aborts because its block DB is unreadable
+    // (a daemon-vs-chaindata format mismatch after an update, or a corrupt index). While set, the
+    // connect loop STOPS crash-restarting into the same abort and offers a one-click reindex instead.
+    bool block_db_reindex_available_ = false;   // node needs its block DB rebuilt (gates restart loop)
+    bool show_block_db_reindex_confirm_ = false; // auto-shown offer dialog
     // Live progress for the switch modal: it stays open from confirm through stop → wait-for-exit → start →
     // reconnect and auto-closes when the new node connects (onConnected). Phase is worker-updated;
     // dialog_open_ gates rendering — both atomic since onConnected/the worker may run off the main thread.
@@ -1324,6 +1329,8 @@ private:
     void renderPinDialogs();
     void renderAntivirusHelpDialog();
     void renderSwitchStopDaemonDialog();  // confirm before stopping an adopted node to switch wallets
+    void renderBlockDbReindexDialog();    // offer to rebuild an unreadable block database (-reindex)
+    void reindexBlockDatabase();          // restart the daemon with -reindex to rebuild the block DB
     void processDeferredEncryption();
 
     // Private methods - connection