feat: add "Start mix" — radio-style infinite playback from any video

[bluegate-studio]

What

Adds a "Start mix" feature that triggers YouTube's algorithmic radio (mix) from any video, via two entry points:

1. Context menu — long-press any video card → "Start mix". Clears the queue, clones the video with a RD-prefixed playlist ID, and opens fresh playback.
2. Player button — a toggle button in the player toolbar. Mutates the current video in-place and reloads suggestions without restarting playback.

How it works

YouTube serves mix playlists when playlistId = "RD" + videoId with playlistIndex = 0 and playlistParams = null. This is the same mechanism the official app uses for "Start Radio".

Context menu path

VideoMenuPresenter → PlaybackPresenter.startMix() — clones the video, sets mix playlist fields, opens via PlaybackPresenter.openVideo().

Player button path

PlayerUIController.onStartMixClicked() — mutates the currently playing Video object in-place, calls loadSuggestions() to fetch the mix chain. No playback restart — the current stream continues while suggestions load asynchronously.

Changes

• VideoMenuPresenter — new "Start mix" menu item with guard
• PlaybackPresenter — startMix() method (clone + open)
• PlayerUIController — onStartMixClicked() handler (mutate + reload)
• PlayerSettingsPresenter — toggle for the player button
• MainUIData — MENU_ITEM_START_MIX constant + default menu order
• PlayerTweaksData — PLAYER_BUTTON_START_MIX constant, enabled by default
• StartMixAction — new TwoStateAction subclass
• VideoPlayerGlue — wires the action into the toolbar
• Resources — action_start_mix.png (192×192, white on transparent), string, ID

Notes

• The player button icon is PNG (not vector XML) because TwoStateAction casts to BitmapDrawable — vectors crash at runtime.
• Enabled by default in both context menu and player toolbar.