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Single source of truth for compact card Tailwind class constants and URL safety validation used across chat-inline `size='sm'` card frames.

## Key Components

### Constants

| Export | Purpose |
|---|---|
| `COMPACT_CARD_OUTER` | Interactive anchor frame with hover/transition styles |
| `COMPACT_CARD_OUTER_STATIC` | Non-interactive span frame (`cursor-default`, no hover) |
| `COMPACT_CARD_SKELETON_OUTER` | Skeleton frame with `animate-pulse` |
| `COMPACT_CARD_IMAGE_SLOT` | 56×56 cover image container |
| `COMPACT_CARD_SKELETON_IMAGE_SLOT` | 56×56 placeholder for skeleton state |
| `COMPACT_CARD_ICON_SLOT` | 56×56 icon container (cards without a cover image) |
| `COMPACT_CARD_TEXT_COL` | Flex column wrapper for title/subtitle/meta rows |
| `COMPACT_CARD_TITLE_ROW` | Fixed 20px title row container |
| `COMPACT_CARD_META_ROW_BOX` | Fixed 16px subtitle/summary row container |
| `COMPACT_CARD_TITLE` | Bold 14px/20px leading title text |
| `COMPACT_CARD_SUBTITLE` | 11px secondary text |
| `COMPACT_CARD_SUMMARY` | 11px secondary text at 80% opacity |
| `COMPACT_CARD_META_ROW` | Multi-part 11px metadata row |
| `COMPACT_CARD_ROW_FILLER` | Non-breaking space to preserve 16px row height |

### Functions

**`safeHref(url)`** — Validates a URL before use in an `<a href>`. Returns the original string only if it passes all safety checks, otherwise `null`.

Allowed:
- `http:`, `https:`, `mailto:` schemes
- Same-origin paths starting with `/` (not `//`)
- Hash fragments starting with `#`

Rejected:
- Control/zero-width/line-separator characters
- Unknown schemes (`javascript:`, `data:`, etc.)
- Bare scheme-only strings
- `http`/`https` URLs with no hostname

## Usage Example

```typescript
import {
  COMPACT_CARD_OUTER,
  COMPACT_CARD_OUTER_STATIC,
  COMPACT_CARD_IMAGE_SLOT,
  COMPACT_CARD_TITLE,
  COMPACT_CARD_SUBTITLE,
  safeHref,
} from './compact-card-classes'

const href = safeHref(card.url)

// Renders an interactive anchor if href is safe, otherwise a static span
const outerClass = href ? COMPACT_CARD_OUTER : COMPACT_CARD_OUTER_STATIC
const Wrapper = href ? 'a' : 'span'
const wrapperProps = href ? { href } : {}
```

> **Card-specific variation** (play overlays, pill badges, status indicators) lives inside each individual card component. These constants govern only the shared outer frame — uniform across all compact card types.