TerraManager v1.13.0 - Guided Start & Interface Polish

TerraManager v1.13.0 makes the first launch easier to understand and polishes
frequently used Shared Care interfaces.

Guided start

• A new skippable first-launch introduction appears for an empty standalone
  collection.
• The tour explains the core Box, Animal, feeding/reminder and QR workflows
  without creating sample data.
• It can be replayed later from Settings.
• Back and Next stay inside the tour; Finish or Skip simply closes it without
  moving the user to another primary page.

Shared Care polish

• Adds a browser-local Show next feeding option in Settings while keeping due
  reminders visible.
• Aligns Shared Care overview controls more closely with the standalone app and
  keeps Big Picture Mode in Settings.
• Reuses unchanged overview pictures while scrolling, sorting and polling.
• Adds a bounded picture cache of up to 128 entries / 32 MiB with four concurrent
  loads, refreshing the snapshot when appropriate and clearing it on exit or
  sign-out.

The result is a lighter first-run experience and substantially less redundant
media traffic during normal Shared Care navigation.

Compatibility: Database Schema Version 16 and Portable Backup Format Version
2 remain unchanged. No additional device permission or external service is
introduced.