# Recurring Schedule Builder

## Objective

Let users build a repeating schedule and see exactly what it will do.

Plain controls for describing recurrence, with a preview of upcoming occurrences and safe editing of a series.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Build the recurrence from ordinary controls — frequency, interval, days, end condition — and store it as a standard recurrence rule rather than an ad-hoc format.
2. Show the next several concrete occurrences, with dates and times, before the user saves. The preview is the feature; a rule nobody can verify is a rule nobody trusts.
3. Support end conditions properly: never, on a date, or after N occurrences.
4. When editing an existing series, ask whether the change applies to this occurrence, this and all following, or the entire series, and store exceptions rather than rewriting history.
5. Do NOT expand the whole series into stored rows up front. Generate occurrences on demand from the rule, or an infinite schedule becomes an infinite table.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Monthly on the 31st must have a stated behaviour in February — skip, or clamp to the last day.
- Yearly on 29 February needs the same decision.
- Splitting a series at 'this and following' must leave earlier occurrences untouched.
- Deleted single occurrences must persist as exclusions and survive later edits to the rule.
- A rule that produces no occurrences at all must be rejected at save time, with the reason.
- The preview must be computed in the schedule's own zone, including across DST transitions.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Recurrence is stored as a standard rule, not free text.
- [ ] The next occurrences are previewed before saving.
- [ ] End conditions cover never, until a date, and after a count.
- [ ] Editing offers this occurrence, this and following, or the whole series.
- [ ] Nonexistent dates such as the 31st of a short month resolve by a documented rule.
- [ ] Occurrences are generated on demand, not materialized indefinitely.
- [ ] Exclusions and skipped dates survive subsequent rule edits.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.