fix(usage): show model and provider for single-item groups

[dajinglingpake]

Problem

Usage summary rows always show — for model/provider fields. When a group contains one model/provider record, no detail row is rendered because detail rows are only available for groups with more than one record, hiding the only model/provider used by that group.

Fix

• Render the unique record's model and provider in the summary row for single-record groups.
• Keep summary placeholders and expandable detail rows for groups with multiple records.
• Apply the behavior consistently to the model, account, API key, and endpoint usage tables.
• Align account summary/detail cells with the table headers.

Validation

• npm run build
• npx eslint src/shared/components/UsageStats.js