Add unified page-based details panel on existing layouts

Phase 1 of the player panels redesign:

- New PanelSettings model (pages, regions, layout mode, default
  visibility) stored as an optional field on PlayerControlsLayout with
  an effectivePanelSettings accessor; lenient Codable so future page
  types and settings payloads never corrupt saved presets.
- New Views/Player/Panels content engine: PlayerPanelContentView
  renders a page bar plus the selected page (info, comments, related,
  chapters, queue, tracks), with runtime hiding of empty pages.
- PortraitDetailsPanel and FloatingDetailsPanel now host the shared
  content instead of their duplicated info bodies; drag machinery,
  pills, and fades are untouched.
- Comments list extracted into CommentsListContent shared by the
  overlay and the comments page; comments pill routes to the comments
  page (expanding the portrait panel) when the page is enabled, and
  falls back to the overlay when it is not.
- Tracks page embeds QualitySelectorView via a new isEmbedded mode.
- Deleted the unused video info bodies in ExpandedPlayerSheet+VideoInfo
  and the dead PlayerInfoTab model.

Yattee/Models/Navigation/PlayerInfoTab.swift

@@ -1,20 +0,0 @@
-//
-//  PlayerInfoTab.swift
-//  Yattee
-//
-//  Player info tab definitions.
-//
-
-import Foundation
-
-enum PlayerInfoTab: String, CaseIterable {
-    case description
-    case comments
-
-    var title: String {
-        switch self {
-        case .description: return String(localized: "player.description")
-        case .comments: return String(localized: "player.comments")
-        }
-    }
-}