Toast notification when auto-network-detect switches networks

[Copexit]

Follow-up from #91 (auto-detect network from txid on NOT_FOUND, merged as da32b55).

Context

When a user pastes a txid that doesn't exist on the currently-selected network, am-i.exposed now silently probes the other public mempool.space networks, switches the selector to the one that holds the tx, and re-runs the analysis. The only visible signal is the network selector chip changing.

PR #91's description flagged this as an open question:

The user is not told that an auto-switch happened beyond the network selector visibly changing. A small toast ("Switched to signet to find this transaction") would be a nice follow-up but feels separable from this fix.

Proposal

Surface a toast (or equivalent transient banner) when the auto-detection fires successfully, e.g.:

Switched to signet to find this transaction.

Notes

• Only triggers on the auto-switch path inside the NOT_FOUND catch in src/hooks/useAnalysis.ts - the explicit user-driven network change should not trigger it.
• Should be localized via i18next - reuse an existing toast component if one already lives in the project, otherwise scope is small enough that a transient inline status above the result card may be simpler than introducing a toast system.
• i18n keys would land in public/locales/{lang}/common.json for all six locales.

Not blocking, polish only.