Align transaction sync asset prefetch

[0xh3rman]

Problem

Opening Activity after a transaction could crash with SQLITE_CONSTRAINT_FOREIGNKEY on the balances -> asset foreign key. Android transaction sync called ensureWalletAssets → enableAsset → linkAssetToWallet(visible = true), inserting a balance row for every transaction-associated asset. When prefetchAssets was partial (the assets API returned fewer/no assets, e.g. on failure), the asset row was never materialized, so the balance insert violated the FK.

This also diverged from iOS behaviorally: enabling with visible = true auto-showed transaction tokens in the wallet list, whereas iOS only stores them hidden.

Fix

Mirror iOS TransactionsService.prefetchAssets → AssetsService.addBalancesIfMissing(newAssets) instead of ensuring/enabling wallet assets:

• PrefetchAssets.prefetchAssets now returns the asset ids it newly persisted.
• Transaction sync inserts a hidden (isVisible = false) balance row only for those newly prefetched assets, via AssetsRepository.addBalancesIfMissing.
• The insert is FK-safe: addBalancesIfMissing filters to assets that exist locally — INSERT OR IGNORE resolves a primary-key conflict but does not suppress the balances -> asset foreign-key violation.

Net result: transaction-associated assets are tracked but stay hidden until the user enables them, matching iOS, and the crash path (enabling unmaterialized assets) is removed.

Verification

• ./gradlew :data:coordinators:testDebugUnitTest --tests "*SyncTransactionsImplTest" --tests "*PrefetchAssetsImplTest" --tests "*EnsureWalletAssetsImplTest"
• ./gradlew :data:repositories:testDebugUnitTest --tests "*AssetsRepositoryTest.addBalancesIfMissing_insertsHiddenBalanceOnlyForExistingAssets"
• ./gradlew :app:testFdroidDebugUnitTest --tests "*NotificationNavigationTest"
• ./gradlew :data:coordinators:lintDebug :data:repositories:lintDebug