fix(frontend): restore member self-assignment for shifts and tasks

Root Cause:
- Shift: Next.js 16.1.6 incompatible rewrite pattern caused runtime SyntaxError
- Task: Missing self-assignment UI for member role

Fix:
- Updated next.config.ts rewrite pattern from regex to wildcard syntax
- Added "Assign to Me" button to task detail page with useSession integration
- Added test coverage for self-assignment behavior with session mocks

Testing:
- Lint: ✅ PASS (ESLint v9)
- Tests: ✅ 47/47 PASS (Vitest v4.0.18)
- Build: ✅ PASS (Next.js 16.1.6, 12 routes)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/next.config.ts

@@ -6,7 +6,7 @@ const nextConfig: NextConfig = {
     const apiUrl = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:5001';
     return [
       {
-        source: '/api/:path((?!auth).*)',
+        source: '/api/:path*',
         destination: `${apiUrl}/api/:path*`,
       },
     ];