Calendar: Evolve post overlay into a Quick Edit modal for enhanced inline editing

## Summary

This issue proposes transforming the calendar's current post overlay (small popup positioned near the calendar cell) into a proper centered modal dialog. This would enable more comprehensive inline editing capabilities, including proper support for hierarchical taxonomies, while solving the performance and usability issues documented in #756.

## Background

Issue #756 identified that the current "editable fields" system has significant problems:

1. **Hierarchical taxonomies don't work properly**: The `wp_dropdown_categories()` single-select dropdown can't represent multiple categories, and would silently remove all but one if it saved successfully
2. **Performance issues**: Loading the full category tree for every post on the calendar causes page bloat
3. **Limited space**: The current overlay (250-350px wide, positioned absolutely near the cell) constrains what can be displayed

PR #757 addressed this by making hierarchical taxonomies non-editable, which is the right immediate fix. However, there's an opportunity to solve this more comprehensively.

## The Proposal: Modal Quick Edit

Replace the current small overlay with a centered modal dialog that:

- **Uses a backdrop overlay** (blurred/dimmed background) following standard modal UX patterns
- **Has adequate space** for multiple editable fields including hierarchical taxonomy checkboxes
- **Loads content via AJAX** to avoid upfront performance cost
- **Follows WordPress patterns** using `@wordpress/components` Modal (already a dependency)

This would create a "Quick Edit" experience similar to the Posts list table, but tailored for the calendar context.

### Current State

```
┌──────────────────────────────────────────────────┐
│ Calendar │
│ ┌─────┬─────┬─────┬─────┬─────┬─────┬─────┐ │
│ │ Mon │ Tue │ Wed │ Thu │ Fri │ Sat │ Sun │ │
│ ├─────┼─────┼─────┼─────┼─────┼─────┼─────┤ │
│ │ │ ┌──────────────┐ │ │
│ │ │ │ Post Title │ ← Small overlay │ │
│ │ │ │ Author: Joe │ (250-350px) │ │
│ │ │ │ Tags: foo │ positioned │ │
│ │ │ │ [Edit|Trash] │ near cell │ │
│ │ │ └──────────────┘ │ │
│ └─────┴─────┴─────┴─────┴─────┴─────┴─────┘ │
└──────────────────────────────────────────────────┘
```

### Proposed State

```
┌──────────────────────────────────────────────────┐
│ Calendar (dimmed/blurred backdrop) │
│ ┌─────┬─────┬─────┬─────┬─────┬─────┬─────┐ │
│ │▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│ │
│ ├─────┴─────┴──┬──────────────────┬────────┤ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ Quick Edit │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ Post Title │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ Author: [▾] │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ Categories: │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ ☑ News │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ ☐ Opinion │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ ☑ Featured │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ Tags: [____] │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │▒▒▒▒▒▒▒▒│ │
│ │▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ [Save] [Cancel] │▒▒▒▒▒▒▒▒│ │
│ ├──────────────┴──────────────────┴────────┤ │
│ │▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│▒▒▒▒▒│ │
│ └─────┴─────┴─────┴─────┴─────┴─────┴─────┘ │
└──────────────────────────────────────────────────┘
```

## What This Enables

1. **Proper hierarchical taxonomy editing**: Room for checkbox trees with indentation, similar to the classic editor category metabox
2. **Multiple editable fields simultaneously**: Author, status, date, multiple taxonomies visible at once
3. **Scrollable regions**: Each taxonomy section can scroll independently if needed
4. **Familiar UX**: Users understand modals - they're used throughout WordPress (media library, block inserter)
5. **Performance**: AJAX-loaded content means no upfront cost; only load full taxonomy trees when user clicks

## Technical Approach

### Existing Infrastructure

Edit Flow already has the building blocks:
- **React components** in `modules/calendar/lib/react/`
- **`@wordpress/components`** (v30.9.0) including Modal component
- **`@wordpress/data`** for state management
- **AJAX save infrastructure** in `calendar.js` for metadata updates
- **`wp_terms_checklist()`** available server-side for hierarchical taxonomies

### Implementation Phases

**Phase 1: Modal Shell**
- Convert existing overlay trigger to open a React Modal component
- Render existing post information inside the modal
- Maintain current Edit/Trash/Preview actions
- No functional changes, just UI restructure

**Phase 2: AJAX Content Loading**
- Load modal content via AJAX when opened (POST ID → modal HTML)
- Add loading state/spinner
- This solves the performance issue - only fetch taxonomy trees on demand

**Phase 3: Hierarchical Taxonomy Editing**
- Re-enable hierarchical taxonomies as editable
- Use `wp_terms_checklist()` to render proper checkbox trees
- Extend AJAX save to handle multiple term IDs
- Add proper capability checks

**Phase 4: Enhanced Fields (Optional)**
- Consider additional editable fields: author dropdown, publication date, post status
- Evaluate based on user feedback from earlier phases

## Considerations

### Scope Management
Once a proper modal exists, there will be requests for more fields (custom fields, featured image, excerpt). Need to define boundaries - this is "Quick Edit", not "Full Edit". The Edit button handles complex operations.

### Mobile/Tablet UX
Modals work well on desktop but need responsive design. The `@wordpress/components` Modal supports `isFullScreen` prop for smaller viewports.

### Accessibility
Modals require:
- Focus trapping (Modal component handles this)
- Escape-to-close (Modal component handles this)
- ARIA labels (need to ensure proper implementation)
- Screen reader announcements

The `@wordpress/components` Modal is designed with accessibility in mind, but we need to ensure proper implementation.

### Performance Trade-offs
- **Pro**: No upfront category tree loading; AJAX loads on demand
- **Con**: Each modal open requires a round-trip
- **Mitigation**: Could cache modal content in JS after first load for same post

## Relationship to Other Issues

- **#756**: This proposal provides a path to properly fix the hierarchical taxonomy editing that #756 identified as broken
- **#757**: The immediate fix (making hierarchical taxonomies non-editable) should ship first; this is a follow-up enhancement
- **#505**: The original caching issue that led to #756's investigation

## Questions for Discussion

1. Should the modal replace the overlay entirely, or should the small overlay remain for "view only" with a "Quick Edit" button opening the modal?
2. What fields should be editable in the initial implementation? Just taxonomies, or also author/status/date?
3. Should we support keyboard navigation between posts (arrow keys to next/prev post while modal is open)?

---

Related: #756, #757, #505
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