Add error handling for token selection

[utgarda]

Problem

Token selection lacks error handling for corrupted sessionStorage data.

Current state

TokenProvider silently ignores JSON parsing errors when loading stored token from sessionStorage.

Solution

1. Handle parsing errors - Clear invalid storage data
2. Add loading state during token initialization
3. Show error messages if token operations fail

Files to modify

• tokenContext.tsx - Add try/catch and error states
• ChainAndTokenSheetPanel.tsx - Show errors in UI

Acceptance criteria

• Handle corrupted sessionStorage gracefully
• Show loading state during initialization
• Display user-friendly error messages