[perf] Replace static inline styles in habit-list-section with CSS classes

[github-actions]

Summary

Replaces 6 static style=\{\{}} objects in habit-list-section.tsx with 4 named CSS classes in habits.module.css. Each inline style object is allocated on every render, contributing to unnecessary garbage collection pressure and preventing the browser from caching the style computation.

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Tier

Tier 2 — Render Performance

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Baseline

Before this change, habit-list-section.tsx contained 6 static inline style objects:

Line	Style	Location
183	style=\{\{ marginTop: 'var(--base-size-12)' }}	(Stack) wrapping action buttons
246	style=\{\{ fontSize: '1.25rem' }}	(Heading) "Active Habits"
264	style=\{\{ fontSize: '1.125rem', marginTop: 'var(--base-size-16)' }}	(Heading) "No active habits yet"
267	style=\{\{ color: 'var(--fgColor-muted)', marginTop: 'var(--base-size-8)' }}	(Text) empty-state description
296	style=\{\{ fontSize: '1.25rem' }}	(Heading) "Completed Habits"
327	style=\{\{ fontSize: '1.25rem' }}	(Heading) "Stopped Habits"

All 6 are fully static — the values never change at runtime.

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Fix

Added 4 CSS classes to habits.module.css:

.actionsStack   { margin-top: var(--base-size-12, 12px); }
.sectionHeading { font-size: 1.25rem; }
.emptyHeading   { font-size: 1.125rem; margin-top: var(--base-size-16, 16px); }
.emptyText      { color: var(--fgColor-muted, #656d76); margin-top: var(--base-size-8, 8px); }
```

Replaced all 6 `style=\{\{}}` props with `className={styles.(class)}`.

The dynamic `style=\{\{ width: \`\$\{progress}%\` }}` on the progress fill div is intentionally untouched — it correctly depends on runtime state.

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## Result

- Zero static inline style objects remain in `habit-list-section.tsx`
- Styles are now static CSS rules, cached by the browser after first parse
- No visual or behavioural change

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## Verification

```
npx tsc --noEmit  ✅ clean
npm run lint      ✅ clean
npm test          ✅ 1025 passed (10 pre-existing auth failures unrelated to this change)

Generated by Daily Performance Improver