fix(spv): clear stale SPV state and banner on backend mode switch

[thepastaclaw]

Issue

When switching from SPV to RPC backend mode, two related issues could leave stale SPV state visible to the user:

1. Stale SPV state leak (Medium): refresh_zmq_and_spv() enters the Rpc branch but never clears spv_no_peers_since or spv_connected_peers. If the mode switch happens without a full reset(), the degraded peer timer persists.

2. Banner not cleared on mode switch (Low-Medium): The SPV degraded banner is only managed inside if core_backend_mode() == Spv. Switching away while the banner is showing leaves it stuck.

Found in code review of #658.

Changes

• connection_status.rs: Clear spv_connected_peers and spv_no_peers_since at the top of the Rpc branch in refresh_zmq_and_spv().
• app.rs: Add an else branch to unconditionally clear the SPV degraded banner when not in SPV mode.

Testing

cargo check passes.

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[thepastaclaw]

Duplicate of #668 — I opened this without checking that Codex had already created a PR. Closing.