Remove @material-ui/core dependency from AccordionCard

## Problem

`@material-ui/core` is used across **4 files** (not just AccordionCard as originally reported):

| File | Imports |
|------|---------|
| `AccordionCard.tsx` | `Collapse`, `IconButton` |
| `EventTextBlocks.tsx` | `Collapse` |
| `ChipInput.tsx` | `Chip` |
| `SegmentSelect.tsx` | `SelectProps` (type-only) |

Additionally, `material-ui-chip-input` has a transitive dependency on `@material-ui/core`.

## Solution

1. Replace `Collapse` with a CSS transition or native ``/`` (2 files)
2. Replace `IconButton` with our own `Button` with `theme="icon"` (1 file)
3. Replace `Chip` with a lightweight styled component (1 file)
4. Replace `SelectProps` type import with our own type definition (1 file)
5. Handle `material-ui-chip-input` transitive dependency
6. Remove `@material-ui/core` from `package.json`

## Acceptance criteria

- [ ] All 4 files migrated off `@material-ui/core`
- [ ] `material-ui-chip-input` transitive dependency handled
- [ ] `@material-ui/core` removed from dependencies
- [ ] No visual regressions

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Part of the Design System Audit (#6606) · Epic: #6882