[perf] Replace inline style in Providers root wrapper with CSS module

[github-actions]

Tier 2 — Render Performance: Eliminate inline style object on root Providers wrapper

Baseline

src/app/providers.tsx contained this inline style on the root (div) that wraps every page in the application:

(div style=\{\{ minHeight: '100vh', backgroundColor: 'var(--bgColor-default)' }})

Because this is a 'use client' component and the object literal is defined inline, JavaScript creates a new object on every render of the Providers tree. This triggers an inline style recalculation in the browser on each re-render, even though the values never change.

Fix

Created src/app/providers.module.css with a static .appRoot CSS class:

.appRoot {
  min-height: 100vh;
  background-color: var(--bgColor-default);
}

Then replaced the inline style with a CSS class reference:

import styles from './providers.module.css';

// Before:
(div style=\{\{ minHeight: '100vh', backgroundColor: 'var(--bgColor-default)' }})

// After:
(div className={styles.appRoot})
```

### Why this improves performance

- The inline style object `{ minHeight: '100vh', backgroundColor: '...' }` was heap-allocated on every render of the Providers component, which runs for **every page in the app** (it's the root layout wrapper).
- A CSS module class is resolved once at build time and referenced as a static string — zero object allocations at render time.
- Browsers apply CSS classes through the CSSOM without triggering inline style recalculations.

### Result

- TypeScript compilation: ✅ Clean (no errors introduced)
- Tests: ✅ 1025/1035 pass — the 10 failures are pre-existing GitHub auth test failures unrelated to this change (`src/lib/github/client.test.ts`, `profile.test.ts`, `workspace-export.test.ts`)
- Bundle size: No change in JS bundle (CSS module class replaces an inline style — net neutral on bundle, small CSS addition)

### Verification

```
npx tsc --noEmit  ✅ No new errors
npm test          ✅ 1025 passed, 10 pre-existing failures unchanged

---

Files changed: src/app/providers.tsx, src/app/providers.module.css (new)

Generated by Daily Performance Improver

[github-actions]

Pull request created: #128