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/client/albums.go

@@ -1,3 +1,4 @@
+// Package client provides methods for interacting with Yandex Music API endpoints.
 package client
 
 import (
@@ -7,7 +8,8 @@ import (
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/utils"
 )
 
-// GetAlbums gets albums by IDs
+// GetAlbums gets albums by their IDs (GET /tracks?album-ids=...).
+// Returns a slice of Album pointers, or nil if no IDs are provided.
 func (c *Client) GetAlbums(ctx context.Context, albumIDs []string) ([]*models.Album, error) {
 	if len(albumIDs) == 0 {
 		return nil, nil
@@ -30,7 +32,8 @@ func (c *Client) GetAlbums(ctx context.Context, albumIDs []string) ([]*models.Al
 	return albums, nil
 }
 
-// GetAlbum gets a single album by ID
+// GetAlbum gets a single album by ID using GetAlbums.
+// Returns an error if no album is found with the given ID.
 func (c *Client) GetAlbum(ctx context.Context, albumID string) (*models.Album, error) {
 	albums, err := c.GetAlbums(ctx, []string{albumID})
 	if err != nil {
@@ -42,7 +45,8 @@ func (c *Client) GetAlbum(ctx context.Context, albumID string) (*models.Album, e
 	return albums[0], nil
 }
 
-// GetAlbumsWithTracks gets albums with tracks expanded
+// GetAlbumsWithTracks gets albums with tracks expanded (GET /tracks?with-tracks=true).
+// Returns a slice of Album pointers with their tracks populated.
 func (c *Client) GetAlbumsWithTracks(ctx context.Context, albumIDs []string) ([]*models.Album, error) {
 	if len(albumIDs) == 0 {
 		return nil, nil