fix(wallet-switch): cluster A — identity/secret teardown on wallet switch

From the wallet-switching audit (HIGH-severity cross-wallet leaks):

- Chat identity leak: the full-node switch (switchToWallet) and seed-migration
  adopt reset state_ but NOT the HushChat identity, so wallet A's decrypted
  conversations surfaced under wallet B and outgoing chat was signed with A's
  keypair. Factor the existing teardown into App::resetChatSession() and call it
  on both wallet-change paths (the lite path already reset it via
  rebuildLiteWallet, which now uses the helper too).
- Global PIN vault: the PIN quick-unlock vault was a single vault.dat, so after
  a switch wallet A's stored passphrase was offered/applied to encrypted wallet
  B. SecureVault is now scoped per wallet (vault-<walletfile>.dat); the default
  wallet keeps the legacy vault.dat for back-compat. vault_ is constructed for
  the active wallet and re-scoped on switch, so B has its own (empty) vault.
- Lock-screen state: switching now clears the carried-over failed-attempt
  counter + lockout timer and secure-zeroes the passphrase/PIN entry buffers so
  the previous wallet's unlock state can't apply to the new one.

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

src/app.h

@@ -658,6 +658,10 @@ private:
     // variants); derives via deriveChatIdentityFromSecret and wipes the secret. No-op when the
     // feature is off, already provisioned, in flight, or unavailable.
     void maybeProvisionChatIdentity();
+    // Drop the in-memory chat identity + decrypted message store, lock the chat DB, and re-arm
+    // provisioning. MUST be called whenever the loaded wallet changes (switch / seed-migration adopt)
+    // so wallet A's private chat can't surface — or be signed with A's keys — under wallet B.
+    void resetChatSession();
     // One-time nudge: on a full-node wallet that has a mnemonic, remind the user (once per
     // install) to back up their seed phrase. Cheap early-outs keep it idle until it can act.
     void maybeRemindSeedBackup();