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/util/i18n.cpp

@@ -1185,6 +1185,14 @@ void I18n::loadBuiltinEnglish()
         strings_["switch_corrupt_body"] = "This wallet appears corrupt — the node couldn't open it. Restore it from a backup, re-create it, or try to repair it.";
         strings_["switch_corrupt_repair"] = "Try to repair (salvage)";
 
+        // Block-database recovery (offered when the node aborts on an unreadable/format-mismatched block DB).
+        strings_["block_db_reindex_title"] = "Rebuild block database?";
+        strings_["block_db_reindex_warn"] = "The node can't read its block database.";
+        strings_["block_db_reindex_body"] = "This usually happens after a daemon update changes the on-disk format, or if the block index is damaged. Your wallet and coins are safe — the node just can't load the chain, so balances show as zero.\n\nRebuilding re-reads your existing block files and can take a while (it also rescans your wallet). Nothing is downloaded.";
+        strings_["block_db_reindex_confirm"] = "Rebuild block database";
+        strings_["block_db_reindex_notify"] = "The node can't read its block database (often after a daemon update). Rebuild it to restore your balance — see the prompt, or Settings › Node.";
+        strings_["block_db_reindex_started"] = "Rebuilding the block database from your blocks — this can take a while.";
+
         // Receive Tab
         strings_["receiving_addresses"] = "Your Receiving Addresses";
         strings_["new_z_shielded"] = "New z-Address (Shielded)";
@@ -1309,6 +1317,7 @@ void I18n::loadBuiltinEnglish()
         strings_["sb_connecting_err"] = "Connecting to daemon — %s";
         strings_["sb_daemon_crashed"] = "Daemon crashed %d times";
         strings_["sb_daemon_start_failed"] = "Couldn't start dragonxd";
+        strings_["sb_block_db_unreadable"] = "Block database unreadable — rebuild required";
         // Persistent node-status banner (App::renderNodeStatusBanner).
         strings_["node_banner_offline_title"] = "Not connected to the DragonX node";
         strings_["node_banner_crashed_title"] = "The node stopped unexpectedly";