feat(seed,console): persistent re-nudging seed backup; confirm destructive console commands

- Seed backup: the one-time 12s toast that marked itself "reminded" the instant
  it fired (so a funded, unbacked wallet was never nudged again) is replaced by
  an actionable alert with a "Back up now" link that opens the backup dialog and
  persists in the bell panel. "Reminded" now means the user actually viewed the
  phrase (set from the backup dialog), so the nudge re-fires each session until
  they do, and escalates to a warning once a balance exists.
- Console: fund-moving / key-revealing commands (dumpprivkey, z_exportkey,
  z_sendmany, z_mergetoaddress, …) now require a confirming re-entry when typed
  or PASTED, honoring the command reference's `destructive` flag that was
  previously enforced only by the reference dialog's Run button. (Output
  masking of key dumps in the scrollback is still a follow-up.)

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

src/app.h

@@ -1406,6 +1406,8 @@ private:
     bool seed_backup_loading_ = false;
     bool seed_backup_no_mnemonic_ = false;
     bool seed_backup_reminder_in_flight_ = false;  // guards the one-time backup nudge probe
+    bool seed_backup_nudged_this_session_ = false; // nudged (or definitively classified) once this session;
+                                                   // unlike the persisted "reminded" flag it re-nudges next run
     std::chrono::steady_clock::time_point seed_backup_reminder_retry_at_{};   // after an inconclusive probe
     bool large_wallet_checked_ = false;            // gate: stat wallet.dat for the bloat nudge once per launch