Add comprehensive godoc documentation to all exported symbols

- Document all exported types, functions, methods, and constants across 52 files
- Add package-level doc comments to client, models, utils, and ynison packages
- Document HTTP method and endpoint path for each API client method
- Document model struct fields and DeJSON/DeList/ToJSON/ToMap methods
- Fix duplicate package doc comment in internal/models/base.go

Co-Authored-By: Claude Code <noreply@anthropic.com>

internal/utils/sign.go

@@ -1,3 +1,4 @@
+// Package utils provides utility functions for the Yandex Music API client.
 package utils
 
 import (
@@ -10,21 +11,23 @@ import (
 )
 
 const (
-	// DefaultSignKey is the signing key from the Android app
+	// DefaultSignKey is the signing key from the Android app.
 	DefaultSignKey = "p93jhgh689SBReK6ghtw62"
-	// DesktopSignKey is the signing key from the desktop app
+	// DesktopSignKey is the signing key from the desktop app.
 	DesktopSignKey = "kzqU4XhfCaY6B6JTHODeq5"
-	// DesktopClient is the X-Yandex-Music-Client header value for desktop
+	// DesktopClient is the X-Yandex-Music-Client header value for desktop.
 	DesktopClient = "YandexMusicDesktopAppWindows/5.95.0"
 )
 
-// Sign represents a request signature
+// Sign represents a request signature for signed API requests.
 type Sign struct {
 	Timestamp int64
 	Value     string
 }
 
-// GetSignRequest creates a signature for a track request
+// GetSignRequest creates a signature for a track request using HMAC-SHA256.
+// It returns a Sign with the timestamp and base64-encoded signature value.
+// If key is empty, DefaultSignKey is used.
 func GetSignRequest(trackID string, key string) Sign {
 	if key == "" {
 		key = DefaultSignKey
@@ -43,7 +46,9 @@ func GetSignRequest(trackID string, key string) Sign {
 	}
 }
 
-// GetFileInfoSign creates a signature for file info requests
+// GetFileInfoSign creates a signature for file info requests using HMAC-SHA256.
+// It returns a Sign with the timestamp and base64-encoded signature value (last character removed as in Python).
+// If key is empty, DesktopSignKey is used.
 func GetFileInfoSign(trackID, quality string, codecs []string, transport, key string) Sign {
 	if key == "" {
 		key = DesktopSignKey
@@ -66,6 +71,7 @@ func GetFileInfoSign(trackID, quality string, codecs []string, transport, key st
 	}
 }
 
+// joinCodecs joins a list of codec strings into a single string.
 func joinCodecs(codecs []string) string {
 	result := ""
 	for _, c := range codecs {