Add n.exchange transaction icon mapping

[EdgeClaws]

Summary

• Add nexchange to the transaction/category plugin icon map
• Add an Unreleased changelog entry for the transaction history/details icon fix

Verification

• git diff --check
• Checked https://content.edge.app/exchangeIcons/nexchange.png; direct HEAD returned 403, so the app-side mapping is in place but CDN asset availability may need reviewer confirmation.

Asana task: https://app.asana.com/0/0/1214071268047343

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Note

Low Risk
Display-only icon mapping with no auth, payment, or swap logic changes.

Overview
n.exchange swaps already use the nexchange plugin elsewhere; transaction history and details were missing a provider icon because pluginIdIcons had no entry for that id.

This PR adds nexchange → nexchange.png on the content server (same pattern as other swap partners) and records the fix in Unreleased changelog.

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[j0ntz]

Superseded by #6079, which carries the same icon-mapping change rebuilt to orch spec. #6079 is the landing target (develop merge + staging cherry-pick per the task's Build field).