Add alternate date/time options for private matches

[Studio-18]

Motivation

• Improve private match invitations by allowing hosts to offer multiple date/time options so invitees have flexible choices and higher chance to join.

Description

• Added alternate/availability option support to the main in-app create flow by introducing availabilityOptions on matchData, UI for adding/removing datetime-local options, client-side validation (no duplicates, not equal to primary time, max 5), and formatted display in the preview and invite screens (src/TennisMatchApp.jsx).
• Implemented the same feature in the dedicated match creator flow with alternateTimes state, date/time inputs, add/remove UI, validation, formatted review list, and sharing text updates (src/components/MatchCreatorFlow.jsx).
• Added helper functions to format alternate times and to build a combined notes payload (buildAlternateTimesNote, formatAlternateTimes, formatAlternateSlots) and included alternate options in notes sent to the API and in calendar/event descriptions and share messages.
• Ensured switching match types clears or preserves alternate options appropriately and surfaced alternate options in share messages and previews so invitees can see backup slots.

Testing

• Started the dev server with npm run dev and confirmed Vite served the app at /ttp-play-dates/ (server ready). (succeeded)
• Ran a headless Playwright script that opened the create flow as an authenticated host, selected a private match, filled a primary date/time, added an alternate date/time option, clicked Add, and captured a screenshot of the alternate options UI; the script completed and produced artifacts/alternate-options.png. (succeeded)
• No unit tests or lint runs were added or executed as part of this change.

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Codex Task

[github-actions]

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