Tennis Coach Platform Frontend

Coach Groups + Lesson Group Selection

This update adds coach-facing player-group management and lesson creation group selection.

• New Groups dashboard tab for listing, creating, editing, and deleting coach groups.
• Group avatar fallback priority: image_url → emoji → initials.
• Group API errors are mapped to user-friendly messages for common statuses (400, 404, 409, 500+).
• Lesson creation now supports selecting groups and sends group_ids with the lesson payload.
• Private lessons now validate that selected groups + selected players + invitees resolve to exactly one player.
• Semi-private and group lessons support mixed manual players + groups with deduped existing player IDs.
• Added utility tests for group-based payload/validation behavior.

Onboarding draft autosave behavior

• On onboarding load, the app fetches canonical onboarding data and a draft in parallel; draft fields override canonical values when present.
• Form edits autosave in the background with a debounced draft PATCH request and status feedback (Saving…, Saved, Save failed. Retry).
• Final submit still uses full onboarding PUT and then refreshes canonical onboarding data.
• A Discard draft action deletes the server draft and restores the form to last saved onboarding data.
• Current onboarding step is persisted locally so refresh restores the same step.