fix(client): make app list keyboard accessible

[shuaiyafei]

Summary

• render frame and app entries as native buttons so they are keyboard focusable and activatable
• expose the current frame and app selection with aria-pressed while preserving the existing styling and callbacks
• add an integration test covering labels, selection state, focusability, and selection callbacks

Validation

• pnpm vitest tests/integration/client/app-list.test.ts --run
• pnpm format:check
• pnpm typecheck
• pnpm lint
• pnpm build
• pnpm test (61 files, 357 tests passed)
• git diff --check

Relates to #885.

This is scoped to AppList.vue and does not overlap with #1146.

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