Add trading history CSV export

[wangxq-cc]

Closes #14.

Changes

• Adds a timeframe selector for History exports: 7 days, 30 days, 90 days, or all.
• Fetches export data through paginated history API calls without mutating the visible history table state.
• Generates a CSV using the same History column labels and value formatting shown in the table.

Validation

• npm run build
• git diff --check

[wangxq-cc]

Note on CI: the Vercel status currently reports Account is blocked, which appears unrelated to this code change. I validated the branch locally with npm run build and git diff --check.