Qt wallet should show correct precision for assets

- [ ] We should get the correct precision from the asset registry
- [ ] We could cache this information in the assetsdir
- [ ] Liquid Qt should show the correct precision if known and satoshis if not known (with some explicit indication when it's not known, perhaps as follows:)
- [ ] We should show the ticker for known assets and no ticker for none.

Start by locating the Qt wallet's asset display entry point and the asset registry integration, then inspect how the assetsdir is used for cached data. Done means known assets display registry precision and tickers, while unknown assets display satoshis with an explicit unknown indication.

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