GUI: Prevent partial asset overview refresh

[Tabakkui]

Problem

When the asset overview refreshes while metadata for a newly received asset is temporarily unavailable, AssetTablePriv::refreshWallet() clears the current model and appends rows while iterating the alphabetically ordered balances. The metadata lookup can then return early, leaving only the prefix processed before the missing asset visible. This matches issue #617's report of the list stopping at the newly registered asset's letter.

Approach

Build the refreshed asset rows in a temporary list and assign it to cachedBalances only after the complete balance iteration succeeds. A transient metadata lookup failure therefore preserves the last complete overview instead of exposing a partial list. A successful refresh, including an empty balance set, still replaces the cached snapshot.

Test plan

• git diff --check
• python3 -m unittest -v tests/issue_617_asset_overview_test.py — 3 tests passed, including a structural regression assertion and a transient metadata-gap simulation
• python3 -m py_compile tests/issue_617_asset_overview_test.py
• Full Qt build and GUI test not run: this ARM64 checkout has no configured build tree or Qt5 development packages; the authorized PC build remains required.

Fixes #617