Fixes two bugs identified by Devin Review on PR #1:
1. History index goes out of bounds (50) when entries exceed the
50-entry cap, causing the first undo to be a silent no-op.
The shift() removed an entry but the index still incremented
past the array bounds.
2. pushHistory uses stale historyIndex closure value inside
setHistory's functional updater, causing entries to be silently
dropped when multiple pushHistory calls are batched by React.
Fix: Combine history entries and index into a single useReducer
state so both are always updated atomically. Add 'reset' action
for new transaction tab creation.
Co-Authored-By: Nakamoto, S <defi@defi-oracle.io>
