fix(rewards): list only the selected child's rewards on a tabbed card (#1074)

[tempus2016]

Fixes #1074

What was wrong

On a rewards card with no child_id pinned, every child tab listed every reward, including ones assigned only to a sibling. With that tab selected, the reward showed the selected child's progress and a Claim reward button. The backend then refused the claim, because the reward is not for that child. This was found while testing v6.4.0 on the dev Home Assistant. The behaviour is older than this release.

Fix

• A new _rewardsFor() lists the rewards open to everyone plus those assigned to the pinned child or, when nothing is pinned, the selected tab. Both the classic and the designed render paths use it in place of their pinned-only filter.
• In the designed styles, the empty state used to replace the whole body, tabs included. The tabs and child selector now stay up when the selected child has no rewards, so another child can still be picked. The classic path already kept its tabs.
• A pinned card behaves as before.

Testing

• 4 new tests in taskmate-rewards-card.test.cjs, run under classic and playroom: a tab lists only that child's rewards, and the tabs stay when the selected child has none. All 4 fail on main.
• Full frontend suite: 721 passed. ESLint is clean and node --check passes.
• Dev Home Assistant: I added a temporary reward assigned only to Vaiha and injected this branch's card. Malia's tab shows 2 rewards without it; Vaiha's tab shows 3 with it, in classic and playroom. Checked by eye in a screenshot. The reward was removed afterwards.