feat(switch): add loading prop (#914)

[wanchun]

Closes #914

Adds a loading Boolean prop to FSwitch that shows a spinner and blocks toggling during async operations.

Implementation:

• switch.vue: loading: Boolean prop, renders LoadingOutlined spinner, blocks click toggle when true
• 5 vitest tests covering: loading icon visible, click blocked, checked/unchecked states, etc.

Files changed: 2. No lockfile. No test-infra changes.

[wanchun]

Closed as part of v3 rebuild from chore/test-infra. Original v1/v2 PRs being superseded by clean re-implementation (vitest + playwright + docs).