fix(security): only a real wrong passphrase counts toward unlock lockout

A failed wallet unlock was always treated as a wrong guess: the lockout
counter was bumped and the back-off curve armed no matter why the RPC
failed. A busy or warming-up node, a transport timeout, or a wallet that
is not actually encrypted would all eat an attempt and could push a user
into a lockout without a single mistyped passphrase.

The failed call is now classified before it is acted on:

- Only the daemon's "passphrase incorrect" error counts as a wrong
  guess and advances the lockout.
- A warming-up or unreachable node, a reply timeout, or a full queue is
  reported as a transient busy-node condition with no attempt charged.
- A "wallet is not encrypted" reply refreshes the local encryption state
  instead of penalising the user.
- Any unknown error surfaces its message without touching the counter.

The PIN path got the same treatment: when the vault decrypts correctly
but the unlock RPC then fails for a non-credential reason, the attempt
is no longer counted.

Tests cover the classification of each failure kind.

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

src/app.h

@@ -910,6 +910,8 @@ private:
     // curve on every failed path (a PIN RPC failure used to skip it).
     void applyUnlockSuccess(const std::string& passphrase, int timeout);
     void applyUnlockFailure(const std::string& errorMessage);
+    // Non-credential failure (busy node, unknown error): message only, no attempt counted.
+    void applyUnlockNotCounted(const std::string& errorMessage);
     // Clear all rescan + witness-rebuild progress/accumulator state. Shared by the four
     // rescan-completion sites so a new witness field can't be forgotten in one copy.
     void resetWitnessRescanProgress();