Check ShipStation postage against FreshBooks expenses

**Objective:** For `{from-date}` to `{to-date}`, compare total ShipStation label costs with shipping expenses in FreshBooks and explain the gap. Default to the last full calendar month when either date is empty.

**Step 1: Label costs (ShipStation server)**

Call `shipstation-get-label` with ship-date.from set to `{from-date}` and ship-date.to set to `{to-date}`. Exclude labels where `voided` is true. Total `shipment-cost` by `carrier-code` and overall.

**Step 2: Booked expenses (FreshBooks server)**

Call `freshbooks-get-expense` with date.from set to `{from-date}` and date.to set to `{to-date}`. Keep expenses whose vendor, account-name or notes mention shipping, postage, freight or a carrier name from step 1. Total `amount` by vendor.

**Step 3: Report**

A table of carrier, ShipStation label cost, FreshBooks expense total and difference, plus overall totals. Say which carriers have costs with no matching expense, and list up to 10 labels (tracking-number and cost) that are not mentioned in any expense note.

**Rules & Guidelines:**

* `shipstation-get-label` is on the shipstation server; `freshbooks-get-expense` is on the freshbooks server.
* Read-only. Do not call `freshbooks-add-expense` or any other freshbooks write tool.
* Expenses and labels will not line up one to one (carriers bill in batches), so compare totals first and only list individual items as hints.

**Validation:**

* If currencies differ between the two sides, report each currency separately and do not add them.
* If there are no labels or no expenses in the period, say which side is empty.