Closes #497 Implemented: feat(docs): add code-sample language selector

[Promise278]

Closes #497
Add a per-endpoint language selector (cURL / JavaScript) to the docs page. Each endpoint now shows a code sample with a radio-group toggle, a copy-to-clipboard button, and session-persisted language preference.

• New CodeSample client component with accessible radiogroup and copy
• New docLanguage helper for sessionStorage read/write with SSR guards
• Endpoint metadata extended with multi-language code samples
• Session-persisted via stableroute.docs.lang sessionStorage key
• Copy-to-clipboard works per active language
• Edge cases: single-language endpoints, corrupt stored values

Coverage: 100% statements, branches, functions, lines on new modules. All 863 tests pass, lint clean, build succeeds.

Summary

Add a per-endpoint language selector (cURL / JavaScript) to the docs page with session-persisted preference and copy-to-clipboard.
What changed:

• New src/components/CodeSample.tsx — Client component with radio-group language toggle, copy button, session persistence via stableroute.docs.lang sessionStorage key
• New src/lib/docLanguage.ts — SSR-safe sessionStorage read/write helpers with fallback logic
• src/app/docs/page.tsx — Endpoint metadata extended with multi-language code samples; renders per endpoint
• src/app/docs/page.test.tsx — Added tests for code samples, radio groups, copy buttons

Key behaviors:

• Languages: cURL, JavaScript per endpoint
• Persisted to stableroute.docs.lang in sessionStorage (session-only, not跨-tab)
• Single-language endpoints show a static label instead of a selector
• Copy-to-clipboard works per active language with toast feedback
• Corrupt/missing stored values fall back to cURL

Edge cases covered:

• Endpoint with one language only (selector hidden)
• Unknown stored language value (falls back to cURL)
• Storage quota exceeded / private mode (graceful fallback)
• Clipboard API unavailable (error toast)

Test results:

• 863 tests pass, lint clean, build succeeds
• 100% coverage on CodeSample.tsx and docLanguage.ts

Type Of Change

• Bug fix
• Feature
• Documentation
• Refactor or maintenance