[perf] Replace inline styles in challenge edit page with CSS module classes

[github-actions]

Summary

Tier: 2 — Render & API Performance

Replaces 7 static inline style=\{\{}} objects in src/app/challenge/edit/[id]/page.tsx with CSS module classes, eliminating per-render JavaScript object allocation on a 'use client' page.

---

Baseline

Before this change, challenge/edit/[id]/page.tsx contained 7 inline style objects that were re-created as new JS objects on every render:

(div style=\{\{ display: 'flex', alignItems: 'center', justifyContent: 'center', minHeight: '400px' }})
(div style=\{\{ maxWidth: '700px', margin: '0 auto', padding: '24px' }})
(Stack ... style=\{\{ marginBottom: '24px' }})
<span style=\{\{ color: 'var(--fgColor-accent)' }}>
(Banner ... style=\{\{ marginBottom: '16px' }})
(Button ... style=\{\{ marginTop: '16px' }})   {/* ×2 */}
(Stack ... style=\{\{ marginTop: '16px' }})

Each style=\{\{}} literal allocates a new object per render in JavaScript. Moving these to CSS module classes makes them zero-cost at runtime (static class strings).

---

Fix

Added 7 new classes to src/app/challenge/challenge.module.css:

Class	Replaces
.editLoadingCenter	display:flex; align-items:center; justify-content:center; min-height:400px
.editFormContainer	max-width:700px; margin:0 auto; padding:24px
.editFormHeader	margin-bottom:24px
.editIconAccent	color:var(--fgColor-accent)
.editActionTop	margin-top:16px (used on 2 error-state buttons)
.editActionsFooter	margin-top:16px (form submit row)
.editSaveErrorBanner	margin-bottom:16px

All values are static (non-dynamic), making this a safe mechanical replacement with zero behaviour change.

---

Result

• 0 inline style=\{\{}} objects remain in edit/[id]/page.tsx (was 7)
• No per-render JS object allocation for layout styles on this page
• CSS module classes are deduplicated and cached by the browser's style engine

---

Verification

• ✅ npm run lint — passes (0 errors)
• ✅ npm test — 1025/1035 tests pass; 10 failures are pre-existing in src/lib/github/client.test.ts, profile.test.ts, workspace-export.test.ts (unrelated to this change)
• ✅ No inline style=\{\{}} remains in the changed file
• ✅ Pure CSS-only change — no logic or behaviour altered

Generated by Daily Performance Improver

[github-actions]

Pull request created: #125