feat(BalanceList): focus management on onAssetClick, search input for filtering assets, sort control toggle in header

[k-deejah]

Problem

1. BalanceList onAssetClick does not manage focus after clicking — when a user clicks an asset row that opens a detail view, focus should move to the detail view's heading or close button. Currently no focus management occurs after onAssetClick fires.

2. BalanceList search/filter input is missing — accounts with 20+ trust lines have no way to search assets by code. A small search input at the top of the card would filter visible rows by assetCode substring match.

3. BalanceList has no sort control beyond the default XLM-first sort — users may want to sort by balance descending (most valuable first) or alphabetically. A sort toggle button in the card header would enable this.

Solution

1. Accept detailRef?: React.RefObject<HTMLElement> in BalanceList; call detailRef.current?.focus() after onAssetClick fires.
2. Add a search <Input> above the asset list that filters rows by assetCode substring.
3. Add a sort toggle button in the card header cycling through "default" | "balance-desc" | "alpha" sort modes.

Acceptance Criteria

• Focus moves to detailRef after onAssetClick
• Search input filters asset rows by code
• Sort toggle cycles through three modes and re-orders the list
• npm run build passes

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Note for Contributors: Write a clear PR description. Include a screen recording of the search filter and sort toggle in action.