[perf] Replace 3 static inline styles in ExportDialog with CSS classes

[github-actions]

Tier 2 — Render Performance: Inline Style Elimination

Baseline

export-dialog.tsx contained 3 static style=\{\{}} objects that created new object literals on every render:

// Loading state wrapper
(Stack style=\{\{ padding: '24px 0' }})

// Visibility label
<span style=\{\{ fontWeight: 600 }}>Repository</span>

// Visibility caption
<p style=\{\{ fontSize: '12px', color: 'var(--fgColor-muted)', margin: 0 }}>
```

### Fix

Moved all 3 static style objects to the existing `ExportDialog.module.css` as named classes:

| Inline style | New CSS class |
|---|---|
| `padding: '24px 0'` | `.loadingContainer` |
| `fontWeight: 600` | `.visibilityLabel` |
| `fontSize: '12px'; color: muted; margin: 0` | `.visibilityCaption` |

### Why This Improves Performance

Static `style=\{\{}}` objects are re-created on every render call, adding GC pressure and bypassing the browser's class-based style cache. CSS module classes are resolved once at parse time and reused across renders with no allocation cost.

### Result

`export-dialog.tsx` now has **zero** inline `style=\{\{}}` props. The `ExportDialog.module.css` module grows by 14 lines (3 new utility classes).

### Verification

```
npx tsc --noEmit   ✅ clean
npm run lint       ✅ clean
npm test           ✅ 1041 passed (10 pre-existing failures in client.test.ts unrelated to this change — require gh auth which is unavailable in CI)

Files Changed

• src/components/ChallengeSandbox/ExportDialog.module.css — 3 new classes added
• src/components/ChallengeSandbox/export-dialog.tsx — 3 inline styles replaced with className references

Generated by Daily Performance Improver

[github-actions]

Pull request created: #53