feat: show pet at regular inbox zero

[salmonumbrella]

Summary

Show the existing animated inbox-zero pet when the regular Inbox or All Inboxes view reaches zero. This reuses the GTD pet implementation so configured pets, fallback behavior, animation, and reduced-motion support stay identical.

Changes

• render the existing GtdZeroPet in the regular inbox empty state
• preserve loading, search, unread-only, no-account, and non-Inbox empty states
• add focused coverage for empty-state visual selection

Testing

• npm test — 1,544 tests passed
• npm run lint
• npm run build

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