Schema guards and a captured-payload contract test for the logbook API

[rfsbraz]

Description

inject.js blindly assumes the logbook response contains pageInfo.rows and data. If geocaching changes the payload shape, the extension throws mid-render (or silently corrupts the table) instead of degrading cleanly.

Proposed Fix

• Validate the response shape before rendering; on mismatch, bail out cleanly into the visible error state (Show a visible error state instead of failing silently #87)
• Commit one real captured logbook payload as a fixture and add a unit test asserting the parser handles it - payload drift then fails CI (complements the live canary in Live-site canary: detect geocaching.com contract drift before users do #86, which catches drift on the wire)