fix(monero-sys): avoid wallet listener handle cycles

[copp1723]

Refs #695

Summary

• Stop wallet event listeners from holding strong wallet handles.
• Downgrade WalletHandleListener and TauriWalletListener captures to Weak.
• Skip deferred store/UI/refresh jobs once the wallet handle has gone away instead of queueing work against a closed wallet channel.
• Return an error from balance_per_subaddress instead of panicking if the wallet channel is closed.

Why

The #695 logs show wallet callbacks queueing store_in_current_file / refresh work around a closed channel, then panicking on closed-channel sends. Weak captures keep callback work tied to live handles only, and the remaining subaddress balance helper now propagates the closed-channel error like the rest of the wallet API.

Verification

• git diff --check
• RUSTUP_TOOLCHAIN=stable cargo fmt --check
• cargo check not fully run locally: monero-sys build script requires the Monero C++ source/submodule and stopped at Target file src/wallet/api/wallet.cpp not found

[Einliterflasche]

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