feat: add settings (volume, seek step, start queue) with TUI/Qt pages

- settings.py persists to .mts-music/settings.json; CLI 'settings' command
- player gains volume support (ffplay -volume / mpv --volume)
- TUI settings screen and Qt settings page (volume, output device, seek step,
  autoplay, start queue, wave-from-history)
- start queue + autoplay applied at startup; arrows use configured seek step
- shared fetch_liked_tracks helper; cache TUI cover renders and Qt scaled cover

settings.py

@@ -0,0 +1,104 @@
+"""Настройки MTS Music: хранятся в .mts-music/settings.json."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from mtsmusic import CONFIG_DIR
+
+SETTINGS_PATH = CONFIG_DIR / "settings.json"
+
+DEFAULTS: dict[str, Any] = {
+    "volume": 80,            # громкость 0..100
+    "seek_step": 5,          # шаг перемотки стрелками, секунды
+    "autoplay": False,       # запускать воспроизведение сразу
+    "start_queue": "wave",   # wave | favorites | history
+    "wave_from_history": True,  # продолжать волну с последних треков
+}
+
+START_QUEUES = ("wave", "favorites", "history")
+START_QUEUE_LABELS = {
+    "wave": "Моя волна",
+    "favorites": "Избранное",
+    "history": "История",
+}
+
+_cache: dict[str, Any] | None = None
+
+
+def _coerce(key: str, value: Any) -> Any:
+    if key == "volume":
+        try:
+            return max(0, min(100, int(value)))
+        except (TypeError, ValueError):
+            return DEFAULTS[key]
+    if key == "seek_step":
+        try:
+            return max(1, min(60, int(value)))
+        except (TypeError, ValueError):
+            return DEFAULTS[key]
+    if key in ("autoplay", "wave_from_history"):
+        if isinstance(value, bool):
+            return value
+        return str(value).strip().lower() in ("1", "true", "yes", "on", "да", "вкл")
+    if key == "start_queue":
+        text = str(value)
+        return text if text in START_QUEUES else DEFAULTS[key]
+    return value
+
+
+def load_settings() -> dict[str, Any]:
+    """Возвращает настройки с дефолтами; файл читается один раз."""
+    global _cache
+    if _cache is not None:
+        return dict(_cache)
+    raw: dict[str, Any] = {}
+    try:
+        data = json.loads(SETTINGS_PATH.read_text(encoding="utf-8"))
+        if isinstance(data, dict):
+            raw = data
+    except (OSError, ValueError):
+        raw = {}
+    merged = dict(DEFAULTS)
+    for key, value in raw.items():
+        if key in DEFAULTS:
+            merged[key] = _coerce(key, value)
+    _cache = merged
+    return dict(merged)
+
+
+def save_settings(values: dict[str, Any]) -> dict[str, Any]:
+    """Сохраняет изменённые настройки и возвращает актуальный набор."""
+    global _cache
+    merged = load_settings()
+    for key, value in values.items():
+        if key in DEFAULTS:
+            merged[key] = _coerce(key, value)
+    _cache = merged
+    try:
+        SETTINGS_PATH.parent.mkdir(parents=True, exist_ok=True)
+        SETTINGS_PATH.write_text(
+            json.dumps(merged, ensure_ascii=False, indent=2),
+            encoding="utf-8",
+        )
+    except OSError:
+        pass
+    return dict(merged)
+
+
+def get_setting(key: str, default: Any = None) -> Any:
+    return load_settings().get(key, DEFAULTS.get(key, default))
+
+
+def set_setting(key: str, value: Any) -> Any:
+    return save_settings({key: value}).get(key)
+
+
+def update_settings(**values: Any) -> dict[str, Any]:
+    return save_settings(dict(values))
+
+
+def settings_path() -> Path:
+    return SETTINGS_PATH