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/pins.go

@@ -1,3 +1,4 @@
+// Package client provides methods for interacting with Yandex Music API endpoints.
 package client
 
 import (
@@ -6,7 +7,7 @@ import (
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/models"
 )
 
-// GetPins gets user's pins
+// GetPins gets user's pins (GET /pins).
 func (c *Client) GetPins(ctx context.Context) (*models.PinsList, error) {
 	var result models.PinsList
 	if err := c.request.Get(ctx, "/pins", nil, &result); err != nil {
@@ -15,7 +16,7 @@ func (c *Client) GetPins(ctx context.Context) (*models.PinsList, error) {
 	return &result, nil
 }
 
-// AddPin adds a pin
+// AddPin adds a pin (POST /pins).
 func (c *Client) AddPin(ctx context.Context, pinType, entityID string) (*models.Pin, error) {
 	params := map[string]string{
 		"type":      pinType,
@@ -29,7 +30,7 @@ func (c *Client) AddPin(ctx context.Context, pinType, entityID string) (*models.
 	return &result, nil
 }
 
-// RemovePin removes a pin
+// RemovePin removes a pin by ID (DELETE /pins).
 func (c *Client) RemovePin(ctx context.Context, pinID string) error {
 	params := map[string]string{
 		"pin_id": pinID,
@@ -38,7 +39,7 @@ func (c *Client) RemovePin(ctx context.Context, pinID string) error {
 	return c.request.Delete(ctx, "/pins", params, &result)
 }
 
-// GetPresaves gets user's presaves
+// GetPresaves gets user's presaves (GET /presaves).
 func (c *Client) GetPresaves(ctx context.Context) (*models.Presaves, error) {
 	var result models.Presaves
 	if err := c.request.Get(ctx, "/presaves", nil, &result); err != nil {