[perf] Replace repeated inline icon styles with shared CSS class in FocusItem cards

[github-actions]

Tier 2 — Render Performance: Inline Styles → CSS Class

Baseline

GoalCard, TopicCard, and ChallengeCard each contained an identical inline style object on the icon-wrapper <span>:

// GoalCard.tsx & TopicCard.tsx
<span style=\{\{ marginRight: '4px', display: 'inline-flex' }}>(CheckIcon size={12} /)</span>

// ChallengeCard.tsx (note: number literal instead of string)
<span style=\{\{ marginRight: 4, display: 'inline-flex' }}>(ClockIcon size={12} /)</span>

Inline style objects are allocated on every render — even though the values never change. Because these cards are rendered multiple times on the dashboard (one instance per goal / topic / challenge), the cost is multiplied per render cycle.

Fix

Added a single shared .iconInline class to the pre-existing FocusItem.module.css (already imported by all three card files):

.iconInline {
  margin-right: var(--base-size-4);  /* 4px — uses design token */
  display: inline-flex;
}

Replaced the three inline style attributes:

<span className={styles.iconInline}>(CheckIcon size={12} /)</span>
```

As a bonus, `ChallengeCard` was using the number `4` instead of the string `'4px'` — the CSS class uses the correct `var(--base-size-4)` token consistently with the rest of the design system.

### Result

- Three style-object allocations eliminated per render cycle (×number of rendered cards)
- Computed CSS is identical — zero visual change
- Aligns with Primer React guidance: prefer CSS classes over `style=\{\{}}`
- `var(--base-size-4)` design token used instead of magic number

### Verification

```
npx tsc --noEmit   ✅ clean
npm test           ✅ 603 tests pass (10 pre-existing failures in client.test.ts require GITHUB_TOKEN — unrelated to this change)

Files changed:

• src/components/FocusItem/FocusItem.module.css — add .iconInline class
• src/components/FocusItem/GoalCard.tsx — use styles.iconInline
• src/components/FocusItem/TopicCard.tsx — use styles.iconInline
• src/components/FocusItem/ChallengeCard.tsx — use styles.iconInline

Generated by Daily Performance Improver

[github-actions]

Pull request created: #29