feat(coding): add native desktop shell

[ktsoator]

Summary

Add a native Wails desktop shell for the Coding product while keeping the existing standalone API and React client usable.

Changes

• host the Coding HTTP/SSE runtime in-process inside a single-instance Wails window
• add graceful runtime shutdown and conversation-manager lifecycle handling
• use the native macOS directory picker with a browser fallback
• integrate the macOS title bar, stable sidebar controls, denser conversation styling, and the product app icon
• preserve the first message until session creation succeeds and surface creation errors
• add Go and Playwright regression coverage plus desktop development documentation

Testing

• gofmt on changed Go files
• go mod tidy
• go vet ./...
• go test ./...
• go test -race ./...
• bun run build
• bun run lint
• bun run test:ui (5 passed)
• wails build

Checklist

• The change is focused and does not include unrelated modifications.
• Tests were added or updated where appropriate.
• Documentation was updated for user-facing changes.
• gofmt, go vet ./..., and go test ./... pass.
• No breaking changes.