fix(app): stop the seed-backup nudge and chat identity retrying every frame

The one-time "back up your seed phrase" nudge only stopped probing on a
success or the exact "not derived from a mnemonic" error. Any other
answer re-armed it for the next frame, so a daemon without the RPC
("Method not found", e.g. the older bundled binary) was sent
z_exportmnemonic at frame rate: ~2,300 requests/s against a test node.
Chat identity setup re-posted its fetch on the next frame after a
transient failure in the same way.

An inconclusive nudge probe now waits 5 minutes before the next; a
transient identity-fetch failure waits 10 seconds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app.h

@@ -769,6 +769,7 @@ private:
     chat::ChatDatabase chat_db_;                  // persistent backing (seed-derived encryption at rest)
     bool chat_identity_provisioned_ = false;      // identity set on the service this session
     bool chat_identity_fetch_in_flight_ = false;  // a z_exportmnemonic worker job is pending
+    std::chrono::steady_clock::time_point chat_identity_retry_at_{};   // after a transient failure
     // Bumped by resetChatSession() on every wallet change; an in-flight identity fetch captures the
     // value at post time and its completion callback discards its (previous-wallet) secret if the id
     // no longer matches — so wallet A's seed can't be provisioned under wallet B via a stale job.
@@ -1098,6 +1099,7 @@ 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
+    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
 
     // Cached mnemonic status of the current wallet, driving the Migrate-to-seed button glow. Probed